One in six inmates in some jails regularly uses heroin and other opiates, with the class A substances surpassing cannabis in popularity among prisoners.
The scale of drug abuse behind bars was uncovered by figures showing that more than 4 per cent of prisoners tested positive for opiates in random tests.
More than 16 per cent of inmates in two jails – Featherstone in Wolverhampton and Erlestoke, Wiltshire – had opiates in their system. Only 38 of the 139 prisons in England and Wales were entirely free of heroin.
Four other jails returned more than 10 per cent of positive opiate results: Peterborough, Lowdham Grange, Nottingham, Lincoln and Altcourse, Liverpool.
The Ministry of Justice (MoJ) disclosed that 4.2 per cent of all tests found evidence of opiate abuse, and 4 per cent of cannabis.
The MoJ said testing for the opiate substitute buprenorphine – also known as Subutex – is to be introduced next month. The survey discovered it was being used in 63 jails.