Prisoners are being charged up to eight times the normal price to telephone their families, a consumer watchdogs will disclose today in a scathing attack on British Telecom and prison chiefs.
Inmates trying to stay in touch with the outside world are being "ripped off" by BT, which has a monopoly over telephone services in English and Welsh jails, according to the National Consumer Council (NCC).
A 10-minute call from inside prison costs nearly three times as much as from a public payphone, while a 60-minute call is eight times as expensive.
For the first time, the NCC is registering a "super-complaint" against named companies – BT and Siemens, which supplies the telephones in 15 Scottish jails. It also condemns the Prison Service, which takes an unspecified share of the money raised by calls, for failing to negotiate a better deal.
The NCC is backing warnings from penal reformers that the prohibitive cost of calls is hitting efforts to rehabilitate offenders.
Juliet Lyon, the director of the Prison Reform Trust, said research had shown that inmates who lost contact with the outside world were six times more likely to return to crime. "Prisoners staying in touch with their families is known to reduce risk, both of reoffending on release and of suicide and self-harm in prison," she said.
"So it is in everyone's interest to enable people to phone home. Prohibitive call charges may make a profit for some, but they do nothing to create a safer society."
Philip Cullum, the NCC's acting chief executive, said: "We don't like seeing vulnerable groups of consumers being ripped off by companies. They don't have any choice and they do seem to be facing unduly high prices."
Inmates buy phone credits in multiples of £1, which they can access through a pin number. Prisoners who want to make foreign calls have to buy a £3 or £5 international phone card.
The NCC said a 10-minute call from prison costs 77p – almost a tenth of the average weekly earnings of an inmate. Because of the tariffs, most inmates are only able to maintain limited contact with their families through brief calls.
BT and the Prison Service struck a 12-year deal to install new phones in jails in 1998. The tariffs have remained static since, despite the average cost of calls to ordinary consumers falling by 60 per cent over the decade. Siemens agreed a similar contract with the Scottish prison service in 2003 based on BT rates.
The NCC said: "We are concerned by the way in which the content and operation of the contracts remain shrouded in secrecy."
It added: "Unlike members of the public, prisoners have no choice about their telephone provider. We consider that this lack of choice should place an obligation on the Prison Service to ensure that prisoners (and their families) are not materially disadvantaged as a consequence."
The "super-complaint" will force Ofcom, the communications regulator, to examine the cost of calls in prison and report back by September. The only other time the NCC resorted to the strongest weapon at its disposal was when it registered alarm over the £2 billion-a-year doorstep lending industry.
Maria Eagle, the Prisons minister, said: "Prices are benchmarked against market conditions. To reduce costs under the current contract would require a large subsidy at public expense."
A BT spokesman said: "The service is not comparable to any other payphone service as it requires a great deal of investment in security and monitoring which is essential to the requirements of the prison service."