The Legal Test: When Is Expert Evidence Required?
The starting point is the rule in R v Turner [1975] QB 834: expert opinion is admissible only where the subject matter lies outside the ordinary knowledge and experience of the court. A judge or jury is assumed to be capable of assessing credibility, drawing inferences from ordinary facts, and applying common sense. Expert evidence is required when the issue is genuinely technical — when the court cannot resolve it without specialist knowledge.
The Supreme Court in Kennedy v Cordia (Services) LLP [2016] UKSC 6 confirmed four conditions for admissibility: the evidence must be necessary to assist the court; it must not be excluded by any exclusionary rule; the witness must be appropriately qualified; and the evidence must be reliable. All four conditions must be satisfied before expert evidence is admitted.
The test for necessity differs across proceedings. In civil proceedings under CPR Part 35, the court applies a proportionality test — is expert evidence necessary and proportionate to the value and complexity of the claim? In family proceedings under FPR Part 25, the Children and Families Act 2014 s.13 imposes a statutory necessity test: the court must be satisfied that the expert evidence is necessary for the proceedings before granting permission. In criminal proceedings under CrimPR Rule 19, no prior court permission is required, but the expert's report must comply with the Rule 19 requirements and be served in accordance with the case management directions.
Ask yourself: is there a factual issue in this case that a judge or jury cannot resolve without specialist knowledge? If the answer is yes, expert evidence is required. The question is not whether expert evidence would be helpful — it is whether it is necessary.
Five Triggers That Indicate You Need an Expert Witness
These are the most common situations in which solicitors and barristers instruct Expert Witness UK. Click each trigger to see what type of expert is required and why.
How the Instruction Process Works by Proceedings Type
The procedural framework that governs expert evidence differs significantly between criminal, civil, and family proceedings. Understanding the applicable framework determines whether you need court permission before instructing an expert, how the report must be formatted, and what funding route applies.
Which Expert Witness Discipline Do You Need?
The most common source of delay in expert witness instruction is selecting the wrong discipline. A case that requires a psychiatrist — because the issue is medical diagnosis of mental disorder — will not be adequately addressed by a psychologist, however experienced. The guide below sets out the specific question each discipline addresses and the proceedings in which they are most commonly instructed.
When the issue involves psychological functioning, cognitive assessment, PTSD, autism, ADHD, or psychological injury.
- Fitness to plead (psychological component)
- PTSD as context for offending
- Autism/ADHD in sentencing
- Psychological injury in civil claims
When the issue requires medical diagnosis of mental disorder, assessment under the Mental Health Act 1983, or a fitness to plead opinion from a medically qualified doctor.
- Fitness to plead (psychiatric component)
- Diminished responsibility
- Mental Health Tribunal proceedings
- Capacity under the Mental Capacity Act 2005
When the issue involves brain injury, epilepsy, neurological conditions, or the neurological basis for a defendant's behaviour.
- Traumatic brain injury and criminal responsibility
- Epilepsy and fitness to drive
- Neurological automatism defence
- Cognitive deficit in sentencing mitigation
When the issue involves physical evidence — DNA, toxicology, pathology, firearms, trace evidence, or document examination.
- DNA evidence
- Cause of death
- Toxicology in drug cases
- Firearms and ballistics
When the issue involves substance dependency, the pharmacological effects of drugs or alcohol, drug-driving, or hair strand testing.
- Drug-driving toxicology
- Dependency as mitigation
- Hair strand testing in child proceedings
- Alcohol dependency and culpability
When the issue involves CCTV footage, digital devices, cell site analysis, social media evidence, or any form of electronic data.
- CCTV identification evidence
- Cell site analysis
- Digital device examination
- Facial mapping
Psychology or Psychiatry? The Most Common Confusion
The single most common question Expert Witness UK receives from criminal defence solicitors is whether they need a psychologist or a psychiatrist. The distinction matters because the two disciplines address different questions, and instructing the wrong one will result in a report that the court cannot use for the purpose intended.
A psychiatrist is a medically qualified doctor who has completed specialist training in mental health. They can diagnose mental disorders under ICD-11 or DSM-5, prescribe medication, assess capacity under the Mental Capacity Act 2005, and give opinions on the relationship between mental disorder and criminal behaviour. A psychologist holds a doctoral qualification in psychology and is HCPC-registered. They assess psychological functioning, cognitive ability, personality, and the psychological impact of events — but they are not medical doctors and cannot make medical diagnoses in the same clinical context.
- The issue is cognitive functioning or intellectual disability
- PTSD, anxiety, or psychological injury is in issue
- Autism or ADHD is relevant to culpability or sentencing
- Psychological assessment of a child is required in family proceedings
- Fitness to plead — psychological component only
- A medical diagnosis of mental disorder is required
- Fitness to plead — full psychiatric assessment needed
- Diminished responsibility under the Homicide Act 1957 s.2
- Mental Health Act 1983 assessment or tribunal proceedings
- Capacity under the Mental Capacity Act 2005
For a detailed comparison of the two disciplines, including the specific assessments each can provide and the legal tests they address, read our guide: Psychologist vs Psychiatrist — Which Expert Do You Need for Court?
LAA Funding and Prior Authority
In legally aided criminal and family cases, expert witness fees are funded through the legal aid certificate. The Legal Aid Agency publishes codified rates for the most common expert disciplines. Where the expert's fee rate falls within the codified rate, no prior authority is required and the expert can be instructed immediately.
Where the expert's fee rate exceeds the codified rate — which is common for consultant psychiatrists, consultant neurologists, and senior forensic scientists — prior authority must be obtained from the LAA before the expert is instructed. The application is made on a CRM4 form and must include a justification for the higher rate, typically by reference to the expert's seniority, the complexity of the case, and the absence of a suitably qualified expert at the codified rate.
Expert Witness UK assists with prior authority applications. Our case managers are familiar with the LAA's requirements and can provide the supporting documentation needed to justify the expert's fee rate. All experts on the panel work within LAA rates wherever possible, and our case managers advise at the outset whether prior authority is likely to be required.
Prior Authority & LAA Rates GuideHow to Instruct an Expert Witness Through Expert Witness UK
Once you have identified that expert evidence is required and determined the correct discipline, the instruction process is straightforward. Expert Witness UK manages the entire process from initial enquiry to report delivery.
Contact our administration team by telephone or through the instruction form. Provide the case type, the specific question to be addressed, the jurisdiction, court deadlines, and funding details (legal aid or private).
Our case managers identify the most appropriate expert from the panel based on the discipline, subspecialty, geographic location, and availability. You receive a CV and full fee quote promptly.
Once you approve the expert, our team coordinates the letter of instruction, the appointment date, and access to medical records and legal papers. Where prior authority is required, we assist with the CRM4 application.
The completed CrimPR Rule 19 compliant report is delivered to your office. We coordinate supplementary questions, joint expert meetings, and court attendance as required.