A three-year criminal investigation into historical abuse allegations at a mental health unit for young people has concluded there are insufficient grounds for a prosecution.

Hertfordshire Constabulary found the use of sedation at the Hill End Hospital Adolescent Unit in St Albans between 1969 and when it closed in 1995 “did not meet the standards of the day”.

More than 100 young patients treated at the unit were spoken to by police, while medical records showed that children were given adult doses in some cases.

Others were repeatedly sedated, the investigation found.

However, there was insufficient evidence to support any arrest or prosecution, including for allegations of sexual assault, police announced on Tuesday.

Detective Superintendent Jerome Kent said: “We know that this has been a long process but we had a fundamental responsibility to all those involved to fully investigate the allegations in relation to Hill End Hospital Adolescent Unit.

“While the investigation is unable to progress further for the reasons outlined, we hope that those who came forward to share their accounts with us will find some closure in the knowledge that they have been listened to, taken seriously, and a full and proper investigation has taken place.”

Emma Jones, a solicitor with law firm Leigh Day representing scores of former patients at Hill End, said: “Although it is disappointing the police have decided not to take this further, the fact that the seriousness of the issues raised resulted in this investigation I hope will go some way to my clients feeling vindicated.

“It is clear from the statement that children were oversedated… children being subjected to what I would say was a ‘chemical cosh’.

“This makes clear that the police found evidence to support these allegations made by my clients and others.”

She said former patients would look into the prospect of a civil claim.