The Government was accused of "criminal negligence" today as judges and magistrates were urged to jail only the most dangerous and persistent criminals in a bid to ease overcrowding.
Ministers' "scaremongering" tactics have blocked prisons with petty offenders, vulnerable women and children, as well as addicts and the mentally ill, according to the Prison Reform Trust.
Director Juliet Lyon said: "Ministers are right to call at last for jails to be used more sparingly, not because they are full to bursting, but because the Government's own scaremongering tactics have blocked prison beds with petty offenders, vulnerable women and children, addicts and the mentally ill," she said.
"The Government has been guilty of criminal negligence to allow prisons to get into such a terrible mess without intervening earlier in a planned way."
Her remarks come after the Home Secretary John Reid, Lord Chancellor Lord Falconer and Attorney General Lord Goldsmith wrote to judges and magistrates in a bid to try to relieve some of the pressure on Britain's overcrowded jails.
The prison population of England and Wales is hovering around its capacity of about 80,000 places.
The Home Office confirmed that a wing with 176 places at Norwich Prison declared "unfit" by inspectors is to be brought back into use from today because of cell shortages.
The BBC has also reported that prison spaces are in such short supply that about 480 people stayed in police cells on Monday and cells in the Old Bailey were also made available this week.
The Home Office has defended the letter to magistrates and judges, saying it was a necessary stopgap measure before plans to create a further 8,000 prison places come into being in the spring.
Mr Reid said: "It is necessary to a civilised society that those who are a danger to our society are put away.
"The public have a right to expect protection from violent and dangerous offenders. Prisons are an expensive resource that should be used to protect the public and to rehabilitate inmates and stop them reoffending.
"However, we should not be squandering taxpayers' money to monitor non-dangerous and less serious offenders."
The letter is understood to be the first in a series of measures designed to ease prison overcrowding over the coming weeks and months.
A Home Office spokesman said the letter had gone out yesterday and was expected to be received by judges and magistrates today.
He said: "The number of prison spaces is now an acute challenge for everyone in the criminal justice system and those who protect the public.
"The first thing the Home Secretary did when he took office was to put in place plans for 8,000 extra places. These are due to come on line from this spring.
"However this means we need to resolve our short-term population issues.
"We are accelerating accommodation arrangements where possible and examining all options for extra capacity in the prison estate as a matter of urgency."
Shadow Home Secretary David Davis said: "It is outrageous that sentences are being dictated by the prison capacity and not by the crime committed.
"Yet again we see the public are being put at risk by the failure of ministers. Offenders who should be sent to jail won't be, and all because the Government failed to listen to our and other calls to address the lack of prison capacity over the last few years.
"How much longer must the public pay the price of Gordon Brown's miserliness and John Reid's incompetence?"
At the end of November last year, the number of prisoners in England and Wales topped 80,000 for the first time in history.
There were 79,908 inmates in jails and a further 152 being held in police stations under the Government's emergency plans, it was announced.
The landmark came as Mr Reid staged a summit with leading experts in a bid to find a solution to the prisons crisis with just 317 spare places left in jails