The official inquiry follows claims from inmates, inspectors and guards, that the switch was taking place because traces of cannabis can remain in the bloodstream for up to a month, compared with a few days for the harder narcotics.
The latest claims about the trend towards harder drugs in jails are made today in a report by Her Majesty's Inspectorate of Prisons.
The inspectorate visited Wayland Prison, a low-security category C jail in Norfolk, where they were told by inmates that use of heroin and crack cocaine was increasing because of the mandatory tests.
Since March, all prisons in England and Wales have been testing for drugs. Ten per cent of the inmates are tested each month. Anyone who tests positive, or refuses to take a test, is punished with loss of privileges and offered a place on a rehabilitation unit.
Wayland was one of the eight pilot prisons chosen for the introduction of mandatory drug-testing programme, which has gradually been introduced across the country since last July as part of a government anti-drugs initiative.
On average, 36 per cent of the inmates tested prove positive. About 90 per cent took cannabis and the remainder a mixture of heroin, cocaine, amphetamines and tranquillisers.
At Wayland, 36 per cent of the 523 prisoners tested were positive. Inmates told the inspectors that drugs remained in the urine for different periods: 30 days for chronic cannabis use and three days for opiates, such as heroin.
"Prisoners had told us that they were aware of these periods, and the use of opiates and crack was therefore increasing," the report states.Reuse content