Prisons in England and Wales will be full to capacity within two months, campaigners predicted, warning that they could have to refuse new inmates this summer because of overcrowding.
A report said the record jail population of 77,774 - just 373 short of the system's capacity - could be passed within weeks.
The Prison Reform Trust said jails had been routinely overcrowded for a decade, and warned that increasing levels of overcrowding were leading to rising rates of reoffending when prisoners are freed. The trust said some jails were holding up to 76 per cent more prisoners than their "certified normal accommodation" or uncrowded capacity. It warned that overcrowding damaged attempts to stop prisoners from returning to crime by offering education and training.
Juliet Lyon, director of the trust, said the figures were a wake-up call to the Government. "Prisons should be places that hold securely, and make every effort to rehabilitate, serious and violent offenders. The skills and focus of those who run them should be wholly directed towards that aim, in the interests of public safety.
"Instead, rapidly rising numbers have reduced many prisons to locked warehouses in which prison officers are called upon to act simply as turnkeys, processing people in transit from overcrowded jail to overcrowded jail.
"No one can be satisfied with a prison system which turns people out more, not less, likely to offend again. Overcrowded prisons are turning petty criminals into the old lags of the future."
The trust said reoffending rates had risen steadily in line with increases in prison overcrowding, and warned that activities designed to prevent prisoners reoffending had been seriously disrupted.
The reconviction rate of former inmates leaving prisons in 2004 was 64 per cent, compared with 51 per cent to 1992. In the same time, the prison population had gone from less than 100 per cent "normal" capacity to 10 per cent overcrowded.
The report said: "The annual Christmas slowdown in processing court cases pulled prison back from the brink of its capacity limit. From the beginning of 2006, the pace began to pick up again. Now the population is 77,004, [within] just a couple of months at the current rate of reaching a new historic high.
"What is now becoming an annual crisis in capacity looks set to hit earlier and harder in 2006. Many people working in the sector are privately predicting the prison system to be entirely full by the summer and even having to consider temporarily closing its doors."
Brian Caton, the general secretary of the Prison Officers' Association, said: "We feel professionally compromised. Because we cannot safely contain and deliver the behaviour programmes that are crucial if our colleagues in the probation services are to tackle offending. I think we will see worse times ahead."
David Davis, the shadow Home Secretary, accused ministers of failing to tackle the shortage of prison places. He said: "The Government should urgently address the lack of prison capacity which it has so far blatantly ignored. They should not seek to continue to deal with this problem by simply letting dangerous offenders out early.
"Prison sentences should be dictated by the severity of the crime, not the prison capacity available. This is demonstrated only too clearly by the recent tragic murders of John Monckton and Mary-Ann Leneghan."
Charles Clarke, the Home Secretary, acknowledged that overcrowding was damaging attempts to cut reoffending rates. He told the BBC: "We have focused very much more on getting proper partnerships to stop reoffending. It will take a very long time to achieve, which is why we have ordered a five-year strategy. What we have to do is to focus the whole system on reoffending and overcrowded prisons, as the Prison Reform Trust say, makes that much more difficult."
A previous overcrowding crisis in 2002 led to inmates being kept in police cells at a far greater cost than normal prison cells.