Government moves to "narrow the justice gap" could see Britain's jail population soar to over 110,000 by the end of the decade, warn official planners.

The figures come amid concerns that a shake-up of the criminal justice system could lead to even greater numbers incarcerated within Britain's prisons.

Home Office forecasters have predicted that an extra 37,000 people could be imprisoned by 2009.

And delivering the required prison capacity could come at the price of two and half pence on the basic rate of income tax.

Plotting a number of scenarios, official research shows the extent of the overcrowding crisis facing the country's jails.

The statistics are based on government crime crackdown targets tagged as "narrowing the justice gap".

If targets are met to clear up 1.2 million crimes by 2006 - an extra 185,200 offences - and if current conviction patterns remain the same Britain's jails will contain between 98,600 and 109,600 prisoners by June 2009.

If custody rates and sentence lengths remain at 2002 levels the prison population is set to climb to 91,400.

The latest figures show that crime crackdowns are straining the prison service - an issue ministers admit has an impact on education and rehabilitation programmes designed to prevent offending.

Prison numbers increased by seven per cent over the year to October 2002.

There was also a rise in the average prison sentence from 21 months to 27.

Following calls to rebalance the criminal justice system in favour of victims, custody rates have soared by 20 per cent in the Crown Courts over the last decade.

And magistrates are now three times more likely to jail an offender than in 1992.

Probation officers warn that "resources for rehabilitation are already at breaking point".

The assistant general secretary of NAPO, Harry Fletcher, believes the price of the penal system could hit government spending plans.

"If the official predicted figures are realised, the state will need to build 49 new jails by 2009," he said.

"This maximum predicted rise in the prison population will cost taxpayers the equivalent of two and a half pence on income tax."

Jail reformers have branded current custody trends a "national shame".

Director of the Prison Reform Trust, Juliet Lyon, said the latest estimates should be a "wake up call" for ministers planning criminal justice reforms.

"They must come as a wake-up call to the government as it rolls out new criminal justice legislation poised precariously between a sensible rebalancing of the system towards effective supervision in the community, and a disastrous drive to lower the custody threshold and propel still more socially excluded people into our overcrowded prisons," she said.

"It is a matter for national shame that we routinely lock up those in need of care or treatment and then release them from our overcrowded jails more, not less, likely to offend again."

The prisons minister, Hilary Benn, argued that the projections highlight the need to move towards community sentencing.

"These projections, based on custody rates and length of sentences currently being handed down by the courts, emphasise why we are developing tougher community sentencing for non-violent and first time offenders," he said.

Benn is sending a "consistent message" to the courts.

"Custody should be used for dangerous, sexual and violent offenders; for others courts, should look at the alternatives," he said.

"We have no intention of compromising public protection, but nor do we advocate any policy that will push up the numbers of those in custody for its own sake."

Minister are bringing 2300 extra prison places on stream with two planned new jails providing another 1300.

The Prison Service's budget of £275 million for 2003 is topped by an extra £60 million "allocated specifically to expand the estate by around a further 700 new places".

"We have also commissioned a review to examine the current management of the prison population and look for longer term answers to stop major increases in the prison population," said Benn.

Ministers acknowledge the system is under pressure.

"The current level of overcrowding in our jails is undesirable, particularly for the 20 per cent of prisoners who are currently having to double up in a cell designed for one," admits Benn.

"We are doing everything we can to ensure that constructive regimes continue to be delivered and are not threatened by an increasing prison population."But Benn makes an appeal to judges to be "sensible" when sentencing.

"However, we need to be assisted in that task by the courts taking sensible decisions about when to use custody."