An expert advisor works for the party who instructs them. An expert witness works for the court, even though one party usually pays the bill. That single difference decides who the opinion is written for, what the other side gets to see, and whether the same person can take both roles in the same case.
In short: an advisor helps a legal team understand the medicine and test its own case, and that advice is normally confidential. An expert witness prepares evidence under Part 35 of the Civil Procedure Rules with an overriding duty to help the court, which displaces any obligation to the party instructing or paying. Moving from the first role to the second is permitted, but it has to be thought through before the instruction is accepted.
Prof. Sandip Hindocha, a Consultant Plastic Surgeon who prepares CPR Part 35 reports, works to the duty in rule 35.3 whenever the instruction is to prepare evidence. What follows sets out where that duty starts and what it does not reach.
What Does an Expert Advisor Actually Do
An expert advisor is instructed to help the legal team, not the court. The work is usually reading the medical records before anyone commits to a position, explaining what the clinical picture will and will not support, and saying plainly whether a pleaded allegation has a medical foundation.
An advisor may also read the other side’s report, or a single joint expert’s report, and help the solicitor work out which parts are worth questioning. None of that is evidence. Nothing is served, nothing is signed with a statement of truth, and the advisor is not named in the directions.
What Changes When an Expert Is Instructed as a Witness
Everything changes at the point the instruction becomes one to give or prepare evidence. Rule 35.2 defines an expert, for the purposes of Part 35, as a person who has been instructed to give or prepare expert evidence for the purpose of proceedings. The definition turns on the purpose of the instruction rather than on the person’s qualifications.
Two consequences follow at once. The report is addressed to the court and not to the solicitor who sent the papers. And under rule 35.4 no party may call an expert or put in evidence an expert’s report without the court’s permission, which has to identify the field of expertise and the issues the evidence will address.
The Duty That Separates the Two Roles
Rule 35.3 is short and it does the heavy lifting. It is the duty of experts to help the court on matters within their expertise, and that duty overrides any obligation to the person from whom they received instructions or by whom they are paid. The wording in Part 35 leaves no room to read it as a tie-break between competing loyalties.
An advisor carries no such duty, because nothing they produce is going in front of a judge. That is the reason the two roles exist separately rather than as two settings on the same instruction.
Where the Official Guidance Draws the Line
The Civil Justice Council’s Guidance for the Instruction of Experts in Civil Claims deals with the point directly. Paragraph 6 says that advice from an expert before proceedings are started, which the parties do not intend to rely upon in litigation, is likely to be confidential, and that the guidance does not apply then. The same applies where, after proceedings have started, experts are instructed only to advise and not to prepare evidence.
Paragraph 7 closes the obvious gap. The guidance does apply if experts who were formerly instructed only to advise are later instructed as an expert witness to prepare or give evidence in the proceedings. Paragraph 9 then restates the rule 35.3 duty in the same terms.
What Does the Other Side Get to See
This is where the difference becomes practical rather than conceptual. Advice a party does not intend to rely on is normally protected. A witness report is served, and it carries more with it than most people expect.
Rule 35.10 requires the report to state the substance of all material instructions, whether written or oral, on the basis of which it was written. Those instructions are not privileged against disclosure, although the court will not order disclosure of any specific document unless there are grounds to question the accuracy or completeness of what the report says about them. The practical effect is that the way an instruction was framed can end up on the page.
Expert Advisor and Expert Witness Side by Side
| Question | Expert advisor | Expert witness |
|---|---|---|
| Who the work is for | The instructing party and its legal team | The court, on matters within the expert’s expertise |
| Governed by CPR Part 35 | No, while the instruction is advisory only | Yes |
| Is the output served | No | Yes, the report is served and disclosed |
| Court’s permission needed | No | Yes, under rule 35.4, to call the expert or put in the report |
| Statement of truth | Not required | Required in the report |
| Takes part in experts’ discussions | No | Yes, where the court so directs under rule 35.12 |
| Can be cross-examined on the work | No | Yes |
Can the Same Expert Do Both Jobs in One Case
It is not forbidden, and paragraph 7 of the guidance assumes it happens. Paragraph 23(d) carries the caution: where an expert advisor is approached to act as an expert witness, they will need to consider carefully whether they can accept a role as expert witness.
The difficulty is a position rather than a rule. An advisor has spent time inside one party’s thinking and may have been asked what helps and what does not. A witness has to be able to say, and to be seen to be able to say, that the opinion would read the same whichever side had sent the papers. Where advisory work went beyond the medicine into how the case should be run, declining the witness instruction is the cleaner answer.
Why a Legal Team Might Use an Advisor First
Rule 35.1 restricts expert evidence to that which is reasonably required to resolve the proceedings. Working out what is reasonably required is hard before anyone has read the records properly, and getting it wrong costs in both directions: the wrong discipline instructed, or a question put that the medicine was never going to answer.
An advisory review is the usual way to answer those questions before the Part 35 instruction is settled. It can establish whether a causation argument is medically arguable, which specialty should be writing, and what the report needs to cover. That makes the eventual instruction narrower, which is what rule 35.1 is asking for.
What Changed When Expert Immunity Was Abolished
Until 2011 an expert witness had immunity from suit for the evidence they gave. In Jones v Kaney [2011] UKSC 13 the Supreme Court removed it, and an expert can now be sued by the party who instructed them over the way their evidence was prepared or given.
That bears on this distinction because it puts both roles on a similar footing for professional exposure while leaving the duties different. An advisor answers to the client for the quality of the advice. A witness answers to the client for the quality of the work and to the court for the independence of the opinion, and the second of those cannot be varied by the letter of instruction.
Experts’ Discussions and Who Takes Part
Rule 35.12 lets the court direct experts to identify and discuss the expert issues and, where possible, reach an agreed opinion. The content of that discussion is not referred to at trial unless the parties agree.
Only instructed experts take part. An advisor can help a solicitor prepare for the discussion and can read the joint statement that comes out of it, but cannot sit in it. A clearly written expert witness report shortens that discussion, because the areas of real disagreement are already visible on the page. If the court then directs the experts to be heard together rather than one after the other, that joint statement also shapes what happens at trial, as set out in our note on hot-tubbing and concurrent expert evidence.
Questions Worth Settling Before the Instruction Goes Out
- Which role is being instructed, advisory or evidential, stated in the letter rather than implied.
- Whether a report is intended to be served, and if so on which issues.
- Whether the expert has advised any party on this matter before.
- Whether the court’s permission has been sought, or will be sought, under rule 35.4.
- Who the report is to be addressed to.
Frequently Asked Questions
Is an expert adviser the same thing as an expert advisor? Yes. Both spellings are in use in England and Wales and the Civil Justice Council guidance itself uses each of them. When people search for expert advisor vs expert witness they are asking about the role, not the spelling.
Does an expert advisor need the court’s permission to be instructed? No. Rule 35.4 bites when a party wants to call an expert or put an expert’s report in evidence. An advisory instruction does neither, so no permission is engaged.
Can advice given by an expert advisor be disclosed later? Advice the party does not intend to rely on is likely to be confidential. That protection is not absolute: privilege can be waived, and once the same expert is instructed to give evidence, the report has to state the substance of all material instructions on which it was written.
Does an expert witness still owe anything to the party paying them? Yes, the ordinary contractual and professional obligations still apply. Rule 35.3 makes the duty to the court override them where the two pull in different directions. Where a court finds the duty was not met, the consequences are set out in our article on when an expert witness is criticised by the court.
Can a single joint expert also advise one of the parties? A single joint expert is defined in rule 35.2 as an expert instructed to prepare a report for the court on behalf of two or more of the parties, so separate advisory work for one of them would cut across the basis of the instruction.
If you are deciding which of the two roles your case needs, our expert witness page sets out the scope of the evidential work, and you can set out the issues and the timetable on the instruct an expert page.
Related reading
This article is general information about the rules governing expert evidence in England and Wales. It is not legal advice and it is not a clinical opinion on any individual case. Anyone dealing with a specific claim should take advice from a qualified solicitor.