Consumer groups said on Tuesday they have made a "super-complaint" to the communications watchdog Ofcom about excessively high costs of making calls from prisons.

The groups said a 30-minute call from a prison to a landline costs seven times more than a similar call from a public phone box. They said the costs are so high that half of all calls from jail last less than three minutes.

"Just because the consumers are prisoners it doesn't mean they should be exploited and pay well over the odds for telephone calls," said Philip Cullum, acting chief executive of the National Consumer Council (NCC).

"A super-complaint is not a move we take lightly and comes on the back of anxieties raised by other official bodies."

The NCC, along with its Welsh and Scottish counterparts, argue that the charges applied by BT in England and Wales and Siemens in Scotland appear unrelated to the cost of providing the service.

They said that the high charges come as other basic phone services have fallen 60 percent.

However BT said the costs could not simply be compared in this way and added that the charges are set in agreement with prison authorities.

"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 specific requirements of Her Majesty's Prison Service," it said in a statement.

Ofcom will now have 90 days to investigate the claims and to make a response.

The Prison Reform Trust, which is backing the complaint, said prisoners are far less likely to reoffend if they have a supportive family network to return to on their release.

"Prisoners staying in touch with their families is known to reduce risk, both of re-offending on release and of suicide and self-harm in prison, so it is in everyone's interest to enable people to phone home," said the Trust's director Juliet Lyon.

"Prohibitive call charges may make a profit for some but they do nothing to create a safer society."