Drug addict prisoners are making special requests to be sent to Wandsworth Prison because drugs are so freely available there, a report has found.

The National Association of Probation Officers (Napo) said drug dealing was rife at the Heathfield Road jail, with inmates able to get "anything they wanted within a very short period of time".

Napo said: "Staff in a hostel in London reported prisoners were expressing a preference to be returned to Wandsworth because they were able to get any drugs they wanted at the end of the corridor."

"Staff in hostels complain the drug activity defeats the purpose of rehabilitation and makes their job that much harder," the report said.

"Others suggested that staff saw the use drugs as a way of keeping a lid on potential disorder."

A former prison governor told Napo the drug testing scheme in Britain's prison system was "deeply flawed".

He said urine samples often became contaminated, or were never even sent off for testing. Prisoners drank massive amounts of water over the weekends to flush drugs out their systems, or exchanged urine samples to avoid being caught.

Prisons Minister David Hanson MP described the report as "remarkably ill-informed".

"All evidence points towards a huge reduction of drug use in prison," he said.

"On average 55 per cent of people coming into prisons can be classified as having a serious drug problem.

"It is not surprising there is an incentive for criminals to smuggle drugs in and equally unsurprising that this puts a lot of pressure on prisons to keep them out."

Mr Hanson said the police worked closely with the Prison Service to address drug dealing and other criminal activity in prisons.