There is "significant" drug use at the centre, the report says

Inspectors said staff at the Colnbrook facility are struggling to cope with the rising number of serious offenders.

The centre near Heathrow Airport contains many foreign criminals waiting to be deported after their sentences.

Inspectors found there were 66 suicide or self-harm attempts in six months plus "significant" drug use.

Diplomatic obstacles

The centre holds detainees deemed too dangerous, unstable or vulnerable to be accommodated elsewhere, including murderers, sex offenders, drugs addicts and arsonists.

Some of those held are co-operating with attempts to remove them from the country but paperwork and diplomatic obstacles are holding them back.

Inspectors found complaints of bullying were not properly investigated and recommendations following a death in 2004 had not been achieved.

They discovered a "significant amount" of drug use, with 91 drug seizures between January and June this year.

The population had become an even more volatile mix, as the number of ex-prisoners and those considered difficult to manage elsewhere swelled Anne Owers Chief Inspector of Prisons

Yet the inspectors found health services were good, residential areas clean and well-maintained and a welfare team was helping prisoners prepare for their release.

The centre was at the "outer limits" of its capacity, staff told inspectors.

Chief Inspector of Prisons Anne Owers said: "In recent months, the population had become an even more volatile mix, as the number of ex-prisoners and those considered difficult to manage elsewhere swelled."

This was compounded by vulnerable detainees from other immigration removal centres being moved to Colnbrook, she said.

"It is essential that the Border and Immigration Agency deals expeditiously with casework and ensures a better balance of detainees as soon as other accommodation becomes available."

A second report on Lindholme immigration removal centre, near Doncaster, south Yorkshire, has also been published.

Action plan

It criticised parts of the centre's health and diversity services, but praised its reception work, education and employment provision.

Lin Homer, head of the Border and Immigration Agency, said the concerns raised about Colnbrook are being taken extremely seriously.

She said: "An action plan addressing the recommendations in today's report is being developed.