The Independent has details of a memo disclosing the plan, which has outraged prison officers and governors, who said court buildings could not provide sufficient washing, catering and recreation facilities.
The memo, "Prison Overcrowding", written by an official of Reliance Custodial Services, a private security firm, said the measure is expected to come into effect next month and last until the end of the year. Prisoners in up to 1,000 court cells will be locked up overnight and at weekends.
Mick Butcher, a sub-area manager, said: "The Home Office have requested prisoner-escort contractors to submit proposals for the detention of prisoners in magistrates' courts cells."
Last night David Roddan, general secretary of the Prison Governors' Association, said: "Prisoners held in magistrates' courts cells will not have access to even a normal basic prison regime. The prison service is having to resort to measures which many professional members of the service would say is unacceptable."
Harry Fletcher, deputy general secretary of the National Association of Probation Officers, said: "The use of magistrates' courts cells has never been considered in the past because they have no toilets, no natural light, and they are very smelly. They are meant to hold people for a couple of hours."
The prison service is so overstretched that it bought four pre-fabricated huts, each containing 40 beds. They are to be set up at Haverigg prison, Cumbria and Kirkham, Lancashire. Last night the service denied there was a crisis. A spokesman said: "We constantly keep our contingency plans under review. We have no firm plans to use any accommodation outside the prison estate."
In the past, police cells were used to cope with prison overcrowding but chief constables now oppose the policy. Closure of police stations has led to a reduction in cells. The cost of the accommodation is more than a night at the Ritz.
The proposal is thought to be the first planned systematic use of cells at magistrates' courts to house prisoners on a long-term basis.
The Prison Officers' Association (POA), which has 29,000 members, said security guards would not be able to cope with 24-hour responsibility. John Boddington, POA chairman, said: "Lives are being put at risk. Our duty is public safety but the priority of the private sector is to make profit for its shareholders."
Paul Cavadino, chairman of the Penal Affairs Consortium, said: "It is hard enough to cope at Christmas in a prison with adequate facilities. The conditions in these cells are unacceptable for a civilised society." The private security firms said they could cope with the extra work. Reliance and Group Four, Securicor and Premier Prison Services, are move thousands of prisoners a week between jails and courts. Their guards get eight weeks' classroom training from Home Office instructors.
Ken Tuckwood, operations director of Reliance Custodial Services, said his staff had been trained as prison officers for private prisons as well as for escort duties. The company is assessing all court buildings in the South-west and Wales, areas in which it has escort contracts, for their ability to provide facilities for prisoners. "If someone is detained for 24 hours or even longer it is not acceptable just to give them a sandwich. We are planning the provision of proper meals," he said.