Crowded jails crisis means sex offenders and robbers will be moved to open prisons

Ministers have ordered the switch to make use of the final 480 places across the entire prison system.

The Daily Mail can reveal they are also begging police to provide extra cells to lock-up convicts as overcrowding reaches crisis point.

But senior police - who are already housing up to 470 inmates each night - say they are unlikely to offer any more help.

With a summer of drunken violence expected, they need their remaining cells for newly-arrested offenders.

If both last-ditch ploys fail, Ministers will be forced to once again turn to the 'nuclear option' of the early release of thousands of convicts reaching the end of their sentence.

Lord Falconer and his deputy, David Hanson, say it would 'damage public confidence' in the criminal justice system.

It is also being resisted by Tony Blair, who does not want early release to be his legacy on law and order.

But if prisons officially run out of space, the executive release of inmates will almost certainly be triggered.

On Friday, the prison population was only 481 sort of capacity - at a record 80,977. Among the total were 415 prisoners being housed in police stations and 24 in court cells.

Of the spaces remaining, most are in open jails - prompting the desperate decision to lift restrictions on who can be sent there.

Any convict sentenced to four years or less is now likely to spend the last two months of their sentence in open prison - including hundreds of sex offenders and robbers.

Previously, only inmates serving sentences of 12 months or less for non-violent or sexual offences were considered under the policy.

The Conservatives have warned against moving higher-risk inmates to open jails, warning it could lead to a wave of escapes.

They said it would place the public, particularly residents near open jails, at heightened risk. There have been more than 7,800 escapes from open jails since Labour came to power, with a quarter of fugitives still at large.

The order to prison governors to transfer the inmates, PSO 24/2007, says: "Occupancy in the open estate is still falling below 90 per cent for much of the time.

"Further measures are therefore necessary to ensure the open estate is fully utilised, subject to the need to protect the public."

Officials said all prisoners would be risk assessed before being moved, and those unsuitable turned down to protect the public.

Negotiations to obtain more police cells have been taking place over the past week. At one stage, 472 convicts were locked in police cells.

Of these 400 were provided under Operation Safeguard at a cost of £400 each night.

The cost to the taxpayer has been around £5m each month since it was first activated last October.

The extra 72 spaces were found under informal arrangements, after the desperate Ministry of Justice asked for help.

The charge per night is likely to be the same or even higher.

A spokesman for the Association of Chief Police officers said: "We are in discussions, and we are doing all we can to make as much space available as possible.

"In reality, we are already providing a high number of cells. If we had more space we would offer it, but we do not."

The bill for keeping an offender in court cells is a staggering £1,800 a night, because private security guards have to be paid overtime to supervise the inmates, and ensure they are properly cared for.

The maximum number of court cells currently available across the country is only 141, and their use has sparked angry protests from governors.

The Prison Governors Association entered an official dispute with the Prison Service for the first time in its history, urging its members not to volunteer to supervise the temporary jails.

But the Ministry of Justice is forcing governors to carry out the task. Juliet Lyon, of the Prison Reform Trust, said: "Imprisonment is now reduced to a dangerous game of musical cells.

"Prison after prison is reaching its safe operating limit so people are being decanted into unsuitable court and prison cells. In some cases, they are stacked up in escort vans reeking of urine. This is no way to run a criminal justice system."

John Reid, the outgoing Home Secretary, promised 8,000 extra prison places before he relinquished control of the system last month, but they will not be ready until 2012.

The Ministry of Justice said: "This instruction (PSO 24/2007) does not imply a relaxation of standards for allocation to open conditions.

"However, there is an expectation that for many prisoners within the scope of this instruction, the level of risk will have been substantially reduced by this stage in their sentence. The normal categorisation and risk assessment process will identify unsuitable prisoners."