Number of prisoners in England and Wales rises by 1,000 in three weeks, piling pressure on justice ministry's figures

A rise of more than 1,000 in the prison population in England and Wales in the last three weeks is threatening to trigger a new round of justice budget cuts, a top Whitehall official has warned.

The latest justice ministry figures published on Friday show that prison numbers have soared by a further 407 in the past week, taking the total population to 87,668 ? 3,500 higher than at the same time last year.

Sir Suma Chakrabarti, the justice ministry's permanent secretary, told MPs that the increase in prison numbers was the biggest rise after the Christmas dip for several years.

He told the Commons public accounts committee that the rapidly rising costs of the prison population was "one of the biggest risks we face".

Chakrabarti, speaking after a fortnight in which the jail population had risen by 300 a week, but before the latest figures, said that the department currently had the funds to meet the extra costs.

But he went on to warn that "if that rate of rise continues we will have to revisit all the figures" in the budget fixed last October. Chakrabarti admitted that the defeats suffered by the justice secretary, Ken Clarke, over his sentencing reforms, including his move to increase the discount for early guilty pleas, had meant that plans to make savings by closing some prisons had been dropped.

"They were changed, so some of those savings are not going to be met through closing capacity necessarily, but through further efficiencies instead," he said.

The renewed rise in prison numbers also deals a blow to Clarke's hopes of reducing and then stabilising the prison population at about 82,000 by the time of the next general election.

The justice ministry is already facing a fierce parliamentary battle over implementing ?350m of legal aid cuts and is pressing ahead with significant savings in the courts, prison and probation budgets. A deeper round of mid-year cuts triggered by the prison crisis would provoke an even bigger political headache for Clarke.

The population figures published on Friday show that 87,688 inmates are being held in a prison system with a "usable operational capacity" of 89,399, giving prison managers an operating margin of 1,721 places.

A further 2,701 prisoners have been released early on electronic tags under the home detention curfew scheme.

Official figures published for the first time this week also show that the jail population has been swollen by 6,162 prisoners serving indeterminate sentences for public protection without a definite release date. More than half of them ? 3,489 ? have already passed their "tariff", the date set by the trial judge for their earliest possible release.

A prison service spokesperson said two new prisons ? Oakwood next to Featherstone prison at Wolverhampton, and Thameside, next to the existing Belmarsh prison in east London ? were due to open to provide more capacity.

"We have seen a significant rise in the prison population since the summer, with very strong rises following the public disorder," she said.

"Managing the increase in the population is challenging but we have been continually developing contingencies to manage the additional population.

"We currently have enough prison places for those being remanded and sentenced to custody. Capacity will continue to increase throughout 2012 with the opening of two new prisons from March. We will continue to explore contingencies arrangements should further pressure be placed on the prison estate."

Juliet Lyon of the Prison Reform Trust said the current jail population was neither justifiable nor sustainable. "In the face of numbers rising at the shocking rate of the size of a small prison a week, for social and economic reasons the government must hold its nerve," she said.

"Instead of revising figures and trying to find scarce funds to pour down the prisons drain, money should be spent on cutting crime. This can be done by working intensively with the serious and violent offenders who should be in prison and for whom there are already sufficient places."

She said reducing the unnecessary use of custodial remands, investing in treating drug and drink addictions and diverting many women and children and the mentally ill from jail would also help.