Part 35 CPR governs the use of expert evidence and assessors in civil proceedings in England and Wales. It sets out when expert evidence may be used, the responsibilities of expert witnesses, the court's control over expert evidence, requirements for expert reports, written questions, single joint experts, discussions between experts, and the consequences of failing to comply with the rules.

The central principle is that expert evidence should be limited to what is reasonably required to resolve the proceedings. An expert's primary responsibility is to assist the court rather than act as an advocate for the party who instructed them.

For solicitors, barristers, claimants, defendants and medico-legal professionals, understanding Part 35 CPR is essential when expert evidence is likely to influence the outcome of a civil claim.

What Is Part 35 CPR?

Part 35 CPR is the section of the Civil Procedure Rules dealing with experts and assessors.

It covers the use of expert evidence in civil litigation and is designed to ensure that expert opinion is relevant, proportionate, independent and useful to the court.

The rules cover:

• The duty to restrict expert evidence
• The overriding duty of an expert to the court
• The court's control over expert evidence
• Written expert reports
• Written questions to experts
• Single joint experts
• Instructions to single joint experts
• Information that parties may be required to provide
• Requirements for expert reports
• Use of another party's disclosed expert report
• Discussions between experts
• Failure to disclose expert evidence
• Expert requests for directions
• Court-appointed assessors

The rules are supplemented by Practice Direction 35, which provides additional requirements concerning expert evidence and the preparation and content of expert reports.

Part 35 CPR and the Expert's Overriding Duty to the Court

One of the most important principles under Part 35 CPR is the expert's overriding duty to the court.

Under CPR 35.3, an expert must help the court on matters within their expertise. This duty overrides any obligation owed to the person who instructed or pays the expert.

This means an expert witness is not supposed to act as an advocate.

An expert should provide an independent and objective opinion, even where that opinion does not support the case of the party who instructed them.

Practice Direction 35 further states that expert evidence should be the independent product of the expert and should not be influenced by litigation pressures. Experts should consider material facts, including facts that may undermine their conclusions. They should also identify matters outside their expertise and explain when insufficient information prevents them from reaching a definite opinion.

The Expert's Role

An expert witness should:

1. Remain independent.
2. Provide objective opinions.
3. Stay within their area of expertise.
4. Consider relevant evidence fairly.
5. Identify limitations in the available information.
6. Explain the reasoning behind their conclusions.
7. Tell the parties if their opinion materially changes.
8. Assist the court rather than advocate for a party.

This distinction is particularly important in medical negligence, personal injury, clinical negligence, psychological injury, rehabilitation and other claims where specialist evidence may be central to determining causation, diagnosis, prognosis or loss.

CPR 35.1: Restriction of Expert Evidence

CPR 35.1 provides that expert evidence must be restricted to evidence reasonably required to resolve the proceedings.

The rule reflects the importance of proportionality in civil litigation.

Parties should not automatically obtain multiple expert reports simply because several specialists could potentially comment on an issue. The evidence should have a genuine purpose in resolving an issue before the court.

For example, a personal injury claim may require expert evidence about:

• The nature and extent of an injury
• Diagnosis
• Causation
• Prognosis
• Future treatment
• Rehabilitation requirements
• Functional limitations
• Psychological consequences
• Long-term disability
• The need for care or assistance

The appropriate expert evidence will depend on the issues that actually require specialist opinion.

CPR 35.4: Court Permission for Expert Evidence

A party generally cannot rely on an expert or put an expert's report into evidence without the court's permission.

This gives the court significant control over expert evidence.

The court may determine:

• Whether expert evidence is necessary
• The issues on which expert evidence may be given
• The number of experts permitted
• The field of expertise required
• Whether a single joint expert should be used
• Whether an expert should attend court
• The scope and cost of expert evidence

Consequently, instructing an expert does not automatically mean that the resulting report will be admissible or relied upon at trial.

CPR 35.5: Expert Evidence in Written Reports

CPR 35.5 establishes that expert evidence should normally be provided in a written report, unless the court directs otherwise.

Written reports allow the parties and the court to understand the expert's opinion before the hearing.

For claims proceeding on the small claims or fast track, the court will not direct an expert to attend a hearing unless attendance is necessary in the interests of justice.

What Must a Part 35 CPR Expert Report Contain?

Practice Direction 35 contains detailed requirements concerning the form and content of expert reports.

An expert report should be addressed to the court, rather than to the solicitor or party who instructed the expert.

The report must include information such as:

• The expert's qualifications
• Relevant literature or other material relied upon
• The substance of material facts and instructions
• Which facts are within the expert's own knowledge
• Details of examinations, measurements, tests or experiments relied upon
• The identity and qualifications of individuals who carried out relevant tests
• Whether tests or experiments were supervised by the expert
• The range of opinion where a genuine range exists
• Reasons for the expert's own opinion

The report must also contain the required statement confirming that the expert understands and has complied with their duty to the court.

Why the Expert's Reasoning Matters

A strong Part 35 CPR report should not simply state a conclusion.

The expert should explain the reasoning that connects the evidence to the opinion.

For example, where an expert is asked whether an accident caused a particular injury, the report may need to consider:

accident → symptoms → clinical findings → relevant medical history → alternative explanations → diagnosis → causation opinion → prognosis

This enables the court to understand not merely what the expert believes but why they have reached that conclusion.

Material Instructions to the Expert

CPR 35.10 requires an expert report to state the substance of all material instructions, whether written or oral, on which the report was prepared.

The purpose is to make the basis of the expert's opinion sufficiently transparent.

However, the rule does not generally mean that every underlying instruction document must automatically be disclosed. CPR 35.10 contains specific provisions concerning when the court may order disclosure or permit questioning about instructions.

Experts should therefore ensure that their reports accurately explain the material instructions relevant to their opinions.

CPR 35.6: Written Questions to Expert Witnesses

A party may put proportionate written questions to an expert instructed by another party or to a single joint expert.

Ordinarily, written questions:

• May be asked only once;
• Must be submitted within 28 days of service of the expert report; and
• Should be limited to clarification of the report.

Unless the court gives permission or the other party agrees, questions should not be used as an opportunity to obtain an entirely new opinion or expand the expert's evidence beyond legitimate clarification.

The expert's answers become part of the expert report.

Failure by an expert to answer properly submitted questions can have consequences. The court may prevent the instructing party from relying on the expert's evidence or may prevent recovery of the expert's fees and expenses from another party.

Single Joint Expert Under Part 35 CPR

CPR 35.7 allows the court to direct that expert evidence on a particular issue should be provided by a single joint expert where two or more parties wish to rely on expert evidence on that issue.

A single joint expert is instructed to prepare a report for the court on behalf of two or more parties.

This approach can be appropriate where the court considers that having multiple experts would be disproportionate or unnecessary.

If the relevant parties cannot agree on the identity of the expert, the court may select the expert from a list prepared or identified by the parties or direct another method of selection.

Instructions to a Single Joint Expert

Where a single joint expert has been ordered, relevant parties may provide instructions.

However, when a party gives instructions, they must send a copy to the other relevant parties at the same time.

The court can also give directions concerning:

• Expert fees
• Expert expenses
• Examinations
• Inspections
• Experiments
• Payment arrangements

Unless the court orders otherwise, the relevant parties are jointly and severally liable for the single joint expert's fees and expenses.

Expert Evidence in Personal Injury and Medical Claims

Part 35 CPR is particularly relevant to personal injury and medical claims where specialist evidence is frequently required.

Examples include:

• Road traffic accident claims
• Workplace injury claims
• Public liability claims
• Clinical negligence
• Medical negligence
• Industrial disease
• Psychological injury
• Brain injury
• Spinal injury
• Orthopaedic injury
• Neurological conditions
• Rehabilitation claims
• Chronic pain
• Amputation
• Psychiatric injury

Depending on the issues, a case may require evidence from an orthopaedic surgeon, neurologist, psychiatrist, psychologist, radiologist, physiotherapist, occupational therapist, rehabilitation specialist or another appropriately qualified professional.

The appropriate expert should be selected according to the issue requiring specialist opinion rather than simply the general subject matter of the claim.

Part 35 CPR and Medical Expert Witnesses

Medical experts working within civil litigation must distinguish their clinical role from their medico-legal role.

A Part 35 CPR medical expert may be asked to address issues such as:

• Diagnosis
• Injury severity
• Causation
• Prognosis
• Treatment
• Future medical needs
• Functional consequences
• Rehabilitation
• Pre-existing conditions
• Acceleration or exacerbation
• Future deterioration

The expert should remain within their professional competence and clearly identify any limitations.

Where the evidence involves several specialist disciplines, each expert should address the questions that fall within their own expertise.

Part 35 CPR and Psychological or Psychiatric Evidence

Psychological and psychiatric evidence may be required where a claimant alleges a psychiatric injury following an accident, illness, negligent treatment or another legally relevant event.

Depending on the issue, a psychologist or psychiatrist may be asked to consider:

• Symptoms
• Diagnosis
• Functional impact
• Causation
• Relevant previous history
• Differential explanations
• Treatment requirements
• Prognosis
• Impact on employment and daily activities

The expert should distinguish between evidence obtained from the claimant, information contained in medical records and the expert's own clinical assessment.

Part 35 CPR and Expert Discussions

CPR 35.12 allows the court to direct experts to discuss expert issues.

The purpose of an expert discussion is to identify areas of agreement and disagreement and, where possible, enable experts to reach an agreed opinion on relevant issues.

The court may specify the issues that the experts must address.

Following the discussion, the court may require the experts to prepare a joint statement identifying:

• Matters on which they agree
• Matters on which they disagree
• Their reasons for disagreement

The content of the experts' discussion itself should not normally be referred to at trial unless the parties agree.

Part 35 Expert Joint Statement

A joint statement can significantly narrow the issues requiring determination.

For example:

Issue	Expert A	Expert B	Position
Diagnosis	Agrees	Agrees	Agreed
Causation	Agrees	Disagrees	Disputed
Treatment	Agrees	Agrees	Agreed
Prognosis	Different view	Different view	Disputed
Future disability	Agrees	Agrees	Agreed

This allows the court to concentrate on genuinely disputed expert issues rather than matters that have already been resolved between the experts.

What Happens if an Expert Changes Their Opinion?

Experts are not expected to maintain an opinion merely because it appeared in an earlier report.

Practice Direction 35 states that if an expert's view changes on a material matter after producing a report, the change should be communicated to all parties without delay and, where appropriate, to the court.

Where an expert significantly alters an opinion, the joint statement should include an explanation or addendum identifying the change and explaining why the opinion has changed.

This reflects the expert's continuing duty to the court.

CPR 35.13: Failure to Disclose an Expert Report

Failure to disclose an expert report can have serious consequences.

Under CPR 35.13, a party that fails to disclose an expert's report may not rely on that report at trial or call the expert to give oral evidence unless the court gives permission.

This makes compliance with court directions and disclosure requirements particularly important.

An expert report should therefore be prepared, finalised, served and disclosed in accordance with the relevant procedural timetable.

CPR 35.14: Expert Requests for Directions

An expert may file a written request for directions where necessary to assist them in carrying out their functions.

This is useful where an expert encounters a procedural issue that cannot appropriately be resolved simply through communication with the instructing party.

Part 35 therefore provides a mechanism for experts to seek assistance from the court where necessary.

Assessors Under Part 35 CPR

Part 35 also deals with assessors.

An assessor may be appointed to assist the court with a matter involving specialist skill or experience. Under CPR 35.15, the assessor may prepare a report or attend a trial to advise the court, depending on the court's directions.

Practice Direction 35 provides additional safeguards concerning proposed assessors, including notification to the parties and an opportunity to object to the proposed assessor or their qualifications.

An assessor has a different role from a conventional expert witness instructed by a party.

Part 35 CPR Compliance Checklist for Expert Reports

Before relying on a Part 35 CPR expert report, parties should consider whether:

• The expert has appropriate qualifications and experience.
• The expert understands their overriding duty to the court.
• The report is independent and objective.
• The expert has remained within their area of expertise.
• Relevant material facts have been considered.
• Material instructions have been identified.
• The expert has explained the reasoning behind their conclusions.
• Relevant literature or other relied-upon material has been identified.
• Any range of professional opinion has been addressed.
• Limitations or uncertainties have been clearly stated.
• The report contains the required declaration concerning the expert's duty to the court.
• The report complies with Practice Direction 35.
• The report has been disclosed in accordance with the applicable directions.

Part 35 CPR and Proportionality

Part 35 CPR should always be considered alongside the broader principle of proportionality in civil litigation.

Expert evidence can be highly valuable, but it can also increase litigation costs and complexity. The objective is therefore not to obtain the greatest possible volume of expert evidence, but to obtain the evidence reasonably required to resolve the issues.

This is why the court may limit:

• The number of experts
• The disciplines involved
• The issues addressed
• The scope of reports
• Expert fees
• Oral expert evidence

The current CPR framework also contains specific provisions concerning the application of Part 35 in different tracks. For example, most of Part 35 does not apply to small claims, subject to specified exceptions including rules concerning restriction of expert evidence, the expert's overriding duty and single joint experts.

Common Part 35 CPR Mistakes

Treating the Expert as an Advocate

An expert should not be instructed to produce evidence designed to support a preferred outcome. Their duty is to assist the court.

Giving Inadequate Instructions

An expert needs sufficient information to address the relevant issues properly. Incomplete instructions can result in an incomplete or unreliable opinion.

Going Beyond the Expert's Expertise

An expert should clearly identify matters outside their professional knowledge rather than provide speculative opinions.

Ignoring Contrary Evidence

Practice Direction 35 requires experts to consider material facts, including matters that may detract from their opinions.

Using Expert Evidence That Is Not Necessary

CPR 35.1 restricts expert evidence to what is reasonably required to resolve the proceedings.

Missing Procedural Deadlines

Failure to comply with disclosure or court directions can prevent a party from relying on expert evidence.

Failing to Address Changes in Opinion

A material change of opinion should be communicated promptly rather than concealed or left unexplained.

Frequently Asked Questions About Part 35 CPR

What does Part 35 CPR cover?

Part 35 CPR covers expert evidence and assessors in civil proceedings. It regulates matters including expert duties, permission, written reports, questions, single joint experts, expert discussions, disclosure and assessors.

What is the main duty of a Part 35 expert?

The expert's overriding duty is to help the court on matters within their expertise. That duty takes precedence over obligations to the party who instructed or pays the expert.

Can an expert act as an advocate for a claimant?

No. A Part 35 expert should provide an independent and objective opinion and should not assume the role of an advocate.

How long do parties have to ask written questions about an expert report?

Ordinarily, written questions must be submitted within 28 days of service of the expert report and must be for clarification unless the court permits otherwise or the other party agrees.

Can the court require a single joint expert?

Yes. Under CPR 35.7, the court may direct that evidence on a particular issue be given by a single joint expert.

What happens if an expert report is not disclosed?

A party that fails to disclose an expert report may be unable to rely on the report or call the expert to give oral evidence unless the court gives permission.

Does an expert report have to be addressed to the court?

Yes. Practice Direction 35 states that an expert report should be addressed to the court rather than the party from whom the expert received instructions.

Can an expert change their opinion?

Yes. An expert may change their opinion where the evidence warrants it. A material change should be communicated promptly to the parties and, where appropriate, the court.

Final Considerations on Part 35 CPR

Part 35 CPR provides the procedural framework for using expert evidence effectively and responsibly in civil litigation. Its emphasis on independence, relevance, proportionality and the expert's overriding duty to the court is central to the proper use of specialist evidence.

For solicitors and other legal professionals, careful selection of experts, clear instructions, compliant reports and effective management of expert discussions can help ensure that specialist evidence addresses the issues genuinely requiring expert determination.