Vulnerable inmates at Maidstone Prison may be used as guinea pigs to test the effects of legal highs, a report has warned.

Published by the Independent Monitoring Board (IMB), the report on how well the prison has met the standards expected of it said use of Spice, a synthetic cannabis , and other psychoactive substances was on the rise and of “great concern”.

There is even evidence that some prisoners test the side- effects of such substances on vulnerable inmates.

It is hoped a law that came into force in May making it a criminal offence to supply or possess psychoactive substances in prisons, and the threat of two years in custody for people caught smuggling packages over prison walls, will help tackle the problem.

Overall, the report was more positive, saying the year ending February 28, 2016, was “a period of continued development”.

However, some issues flagged up by previous reports persist, like the provision of dental care, which is still described as inadequate, with 130 inmates on a waiting list for treatment.

Some inmates – roughly 12 to 15 at any one time – are still remaining in the prison after completing their sentences because of issues with their deportation.

The report says this includes those who “are violent and likely to re-offend but may be released into the community as there is no potential for deportation.”

The IMB has asked the Ministry of Justice what’s being done to address this and said prisoners should have the same access to legal advice as people in immigration remand centres.

Maidstone is a category C prison with a capacity of 600 inmates and only houses foreign nationals.

The IMB is a nine-person committee comprising volunteers from the town and reports annually to the Secretary of State.

A Prison Service spokesman said: “As this report recognises, improvements have been made. But we must do more to tackle drugs in our prisons.

“That’s why we’ve put a range of robust measures in place, including the use of search dogs and tough new laws which will see those who smuggle packages over prison walls, including new psychoactive substances, face up to two years in prison.”