Expert witness guide

This guide is for expert/skilled witnesses. It explains the standard of evidence the court requires and your duties regarding revelation of relevant evidence.

The role of expert/skilled witnesses

An expert or skilled witness is a person who through practice, or study, or both, is specially qualified in a recognised branch of knowledge.

Expert or skilled witnesses come from a wide range of professional backgrounds. Some common ones include:

  • engineering
  • finance
  • forensics
  • IT
  • medicine
  • science

Experts are the only type of witness whose opinion carries weight in court. We involve expert witnesses in court cases because of your specialist technical insights. Your evidence brings to the justice process what Crown and defence legal teams cannot do alone. As such, expert evidence is valuable and sometimes critical.

Qualifications, experience, and expertise are important, but not always necessary. Ultimately, it is the court that decides if a witness will be treated as an expert.

What is required in your evidence

Duty to the court

Your principal duty is to the court, not the prosecution or defence. Evidence must come from the independent thinking of the expert witness. It must not be influenced by those who instructed them. Expert evidence is objective and unbiased.

Witnesses should highlight what is fact and what is assumption. Do not leave out facts that go against your conclusions.

Subject matter

Make it clear when any questions or subject matter are outside of your expertise. State any weaknesses in research or data and declare if your opinion is therefore just a provisional one.

Write your subject matter so that a judge and jury with no prior knowledge of your field can follow it.

Show, if possible, your ‘working’ or logical steps that led you to your conclusions.

Justifying conclusions

Explain why any relevant material has been ignored or rejected from your conclusions. In your expert report, cite any external information sources as references. The report should also list your professional experience, accreditation, and any relevant qualifications.

What carries weight is your reasoning – not your conclusion. Show evidence to justify your conclusions.


Expert witnesses must reveal to the Crown all relevant information about their investigations. This obligation takes legal priority over any other you may have, including to your professional body.

Disclosure of relevant material to the accused – in advance – is fundamental to a fair trial.

The Crown has a disclosure responsibility to the accused, which it cannot meet unless you meet yours to the Crown.

What you need to reveal to the Crown

All expert witnesses should read the Disclosure Manual. It details your duties. You will have to sign a declaration to confirm you have read, understood, and followed its instructions.

Remember the four ‘R’s of disclosure:


Keep everything, including physical, handwritten, and digital material, until told otherwise.


Keep records of all the work you carry out and any findings you make in relation to the investigation. You must start this from when you receive instructions. Continue until the end of your involvement in the case.


Make the Crown aware of all relevant material you have in your statement or report. Your records should be available to the defence if requested. This includes information that strengthens the defence case and weakens the Crown’s.

Supporting work

Where other persons assisted you in expert work, reveal this to the prosecutor. Also give warning about published work that you wish to quote in your evidence. The Crown needs to know in advance so this can be made available in court.

Failure to reveal

If you fail to disclose information relevant to the case, it may:

  • halt or delay the prosecution
  • cause a conviction to be overturned
  • undermine your credibility as an expert witness
  • result in action by your professional body or the accused against you

However, a correct approach to evidence and disclosure can enhance your credibility as an expert.

How to deal with omissions

Contact COPFS if you later realise that you omitted relevant material from what you revealed. You should then submit two items:

  • Supplementary report
  • National Standard Statement


Review your conclusions if any new information comes to light, both before and after a trial or appeal. Any information you previously deemed to not be relevant may now be relevant.

When to reveal

You will reveal when you submit your report or statement.

Also, if new, relevant information comes to light at any stage, you should reveal it to the Crown. This could be before, during, or even after the trial.


Precognition is when we meet witnesses before the trial. We find out what you plan to say in evidence, and put it together into a statement.

If we discover gaps in your conclusions at this stage, it means we can ask you to go and work on supplementary findings, ready for court.

You may also want to discuss your method of presentation, so the courtroom can be set up appropriately.

At the precognition meeting, tell us your preferred dates and times to attend court. It may be possible to schedule the giving of evidence so it is more convenient for you.

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