Criminal legal aid solicitors struggling to make ends meet cannot afford to shoulder the cost of intermediaries to help with vulnerable clients in the police station while they wait for government approval, an influential human rights group has warned.

Intermediaries help vulnerable victims, witnesses, suspects and defendants give complete, coherent and accurate evidence to the police and to courts. The National Institute for Health and Care Excellence estimates that three in four people detained in police custody have a mental health issue.

Justice's Mental Health and Fair Trial report, published today, says solicitors must currently cover the costs of an intermediary while they wait for Legal Aid Agency approval for the costs of the intermediary assessments and other pre-trial work. 'This cost risk makes many solicitors, who already struggle with small margins under their legal aid contracts, reluctant to request intermediaries even if they are aware of their benefit,' the report says.

Justice says the Ministry of Justice should extend its registered intermediary scheme for witnesses to suspects and defendants. Intermediaries should also be 'embedded' in the police station, on a duty scheme basis, so they can provide immediate help.

Andrea Coomber, director of Justice, said the organisation has 'long been concerned' that the criminal justice system is not suitably designed to accommodate people with mental health or learning difficulties.

She added: 'There are still fundamental problems with the criminal justice system's response to vulnerability and too few people receive reasonable adjustments to enable them to effectively participate in their defence.'

Justice makes 52 recommendations. It suggests that mental health experts, not police officers, should be responsible for identifying people with vulnerability as a result of mental ill health or learning disability. A specialist prosecutor should be appointed for each Crown Prosecution Service area, responsible for making charging decisions for cases involving vulnerable people. Courts should have a dedicated mental health judge as well as a capacity-based test of fitness to plead and fitness to stand trial. The Sentencing Council should draft a guideline on mental health and vulnerability.

Lord Burnett, the lord chief justice, told a Justice event last night that the report 'provides a rich seam of material...for policymakers and others. I am sure it will play a leading role in developing our approach to vulnerable defendants and witnesses'.