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Procedural Guide

Letter of Instruction to Expert Witness

A compliant letter of instruction is the foundation of every expert witness instruction. Get it wrong and the expert's report may be excluded, the instruction may be challenged, or the court may draw adverse inferences. This guide covers what every letter of instruction must contain, how requirements differ across criminal, family, and immigration proceedings, and the most common drafting errors that solicitors make.

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What Is a Letter of Instruction?

A letter of instruction (LOI) is the formal document by which a solicitor commissions an expert witness to prepare a report for use in legal proceedings. It defines the scope of the expert's retainer, sets out the factual background, poses the questions the expert must address, and confirms the procedural framework within which the expert is operating.

The letter of instruction is not merely an administrative formality. Under CPR Part 35.10(3), instructions to an expert are not privileged against disclosure in civil proceedings. In family proceedings, the letter of instruction is routinely filed with the court and seen by all parties. A poorly drafted letter will produce a poorly scoped report — and a report that does not address the right questions is of limited value, however well written.

The procedural rules that govern expert evidence — CrimPR Rule 19, CPR Part 35, and FPR Part 25 — do not prescribe a standard form for letters of instruction. What they do prescribe is what the expert's report must contain. The letter of instruction must therefore be drafted with those requirements in mind, so that the expert can produce a compliant report.

Mandatory Content in Every Letter of Instruction

Regardless of the type of proceedings, every letter of instruction to an expert witness should contain the following elements.

Identification of the proceedings
The court, the case reference, the parties, and the nature of the proceedings (criminal, family, or immigration). Include the next hearing date and the date by which the expert's report is required.
The instructing party
The name and contact details of the instructing solicitor, the firm, and the client. In joint instructions, identify all instructing parties and nominate a lead solicitor for communications.
The expert's overriding duty
Confirm in express terms that the expert's overriding duty is to the court, not to the instructing party. This is required by CrimPR Rule 19.2, CPR Part 35.3, and FPR Part 25.3. The expert must understand that this duty overrides any obligation to the party who instructed them.
Factual background
A clear, accurate, and balanced summary of the relevant facts. Do not present only the facts that support your client's case. The expert will form their own view of the facts, and a one-sided summary may undermine the independence of the report.
The specific questions
Set out the precise questions the expert is asked to address. Vague instructions produce vague reports. Each question should be capable of being answered within the expert's area of expertise. Avoid questions that call for a legal conclusion — that is the court's function.
Documents provided
List every document provided to the expert. Include all relevant medical records, witness statements, police disclosure, and previous expert reports. The expert must state in their report the documents they have considered.
Format and length
Specify whether a full report or a short addendum is required. State the maximum length if relevant. In criminal proceedings, the report must comply with CrimPR Rule 19.4. In family proceedings, it must comply with FPR PD25B.
Fee arrangement
Confirm the agreed fee, the basis of charging (hourly rate or fixed fee), and the LAA authority if the matter is publicly funded. In LAA-funded matters, prior authority should be obtained before instructing the expert.

Letters of Instruction in Criminal Proceedings

In criminal proceedings, the defence solicitor instructs an expert independently of the prosecution. The expert's report is covered by legal professional privilege until the defence decides to rely on it. If the report is served on the prosecution, it becomes disclosable and must comply with CrimPR Rule 19.

The letter of instruction in criminal proceedings should address the specific issues that arise in the case. For a psychiatric or psychological expert, those issues will typically include fitness to plead, mental disorder at the time of the offence, diminished responsibility, sentencing mitigation, and dangerousness. The letter should provide the defendant's full antecedents, the prosecution case summary, all relevant medical and social services records, and any previous psychiatric or psychological reports.

Where the matter is LAA-funded, the solicitor must obtain prior authority before instructing the expert. The LAA will not pay for expert fees that exceed the prior authority limit unless exceptional circumstances are demonstrated. The letter of instruction should therefore confirm the authorised fee and the basis on which it was calculated.

CrimPR Rule 19 — Key Requirements for the Expert's Report
  • State the expert's qualifications and experience relevant to the opinion expressed
  • Identify the documents and other information considered
  • State the facts and assumptions on which the opinion is based
  • Identify any matter that falls outside the expert's area of expertise
  • Distinguish between concluded opinions and provisional ones
  • Include a statement that the expert understands their duty to the court

Letters of Instruction in Family Proceedings

Family proceedings operate under a more prescriptive regime. Under s.13 Children and Families Act 2014, expert evidence in children proceedings may only be admitted where it is necessary to resolve the proceedings justly. The court must give permission before an expert is instructed, and the letter of instruction must comply with FPR PD25B.

FPR PD25B requires the letter of instruction to contain: the nature of the proceedings and the issues in dispute; the relevant background; the specific questions for the expert; the documents provided; the timetable for the proceedings; the date by which the report is required; and the fee arrangement. The letter must be agreed between the parties before it is sent to the expert. If the parties cannot agree, either party may apply to the court for directions.

In public law children proceedings, the 26-week timetable under s.32 Children Act 1989 imposes strict deadlines. The letter of instruction must be sent promptly after the court grants permission for the expert to be instructed, and the expert must be given a realistic but tight deadline for delivery of the report.

FPR PD25B — Additional Requirements for Family Proceedings
  • The letter must be agreed by all parties before it is sent
  • A copy must be filed with the court
  • The timetable for the proceedings must be included
  • The letter must identify the issues in dispute that the expert is asked to address
  • The expert must be told whether they are a single joint expert or a party-appointed expert

Letters of Instruction in Immigration Proceedings

Immigration tribunal proceedings are governed by the Tribunal Procedure (First-tier Tribunal) (Immigration and Asylum Chamber) Rules 2014. There is no equivalent of CPR Part 35 or FPR Part 25 in the immigration context, but the Upper Tribunal has issued guidance on the use of expert evidence in asylum cases.

The letter of instruction in immigration proceedings should set out the appellant's immigration history, the basis of the asylum or human rights claim, the specific issues the expert is asked to address, and all relevant documents including the Home Office refusal letter, the appellant's witness statement, and any country guidance caselaw relevant to the claim.

For Istanbul Protocol assessments in asylum cases, the letter of instruction should ask the expert to assess whether the appellant's physical and psychological presentation is consistent with their account of torture or ill-treatment, applying the Istanbul Protocol methodology. The expert should be provided with the appellant's full medical records and any previous medico-legal reports.

Common Drafting Errors

These are the errors that most frequently cause problems with expert witness reports.

Vague or over-broad questions
Asking the expert to 'comment on the defendant's mental health' produces a report that addresses everything and answers nothing. Each question must be specific and capable of a precise answer.
Questions that call for a legal conclusion
The expert cannot be asked whether the defendant is guilty, whether the threshold criteria under s.31 Children Act are met, or whether the claimant has a valid asylum claim. Those are legal questions for the court.
Failing to provide all relevant documents
An expert who has not seen all the relevant records cannot give a complete opinion. Failing to provide documents — whether by oversight or deliberate selection — will undermine the report and may lead to it being excluded.
Omitting the overriding duty statement
Every letter of instruction must confirm the expert's overriding duty to the court. Omitting this statement is a procedural error that may lead the court to question the independence of the report.
Instructing without LAA authority
In publicly funded matters, instructing an expert without prior authority from the LAA creates a risk that the fees will not be paid. The LAA will not retrospectively authorise fees that exceed the standard rate unless exceptional circumstances are demonstrated.
Setting an unrealistic deadline
A good expert will decline an instruction if the deadline does not allow sufficient time for a thorough assessment and report. Setting an unrealistic deadline produces a rushed report — or no report at all.

Instructing a Single Joint Expert

Where the court directs that evidence be given by a single joint expert (SJE), the letter of instruction must be agreed by all parties. The process for agreeing the letter of instruction is as follows: the claimant or applicant's solicitor prepares a draft; the other parties have an opportunity to comment and propose amendments; if agreement cannot be reached, the court will resolve the dispute on application.

The SJE owes duties to all parties equally. The letter of instruction must reflect this by presenting the factual background in a balanced way and by including questions from all parties. Either party may put written questions to the SJE after the report is received, under CPR Part 35.6.

In family proceedings, the court will almost always require a single joint expert in children cases. The lead solicitor (usually the applicant's solicitor) is responsible for sending the agreed letter of instruction and for filing a copy with the court.

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