The prison population could increase 25% by 2007 if present trends continue.

Figures released by the Home Office on Friday suggest inmate numbers could rise from 65,000 to 80,300 within seven years, meaning ten new prisons would need to be built.

The projections take into account tougher sentencing policies brought in by the government and a lower-than-expected take-up of electronic tagging.

The 80,300 figure is the top end of projections based on whether current sentencing trends, including how many are jailed and for how long, continue.

Under other scenarios, the figures could rise to between 70,400 and 74,400.

Tougher sentences

Factors set to push up the number of inmates include the introduction of "three strikes" sentencing for burglars, under which third-time offenders will automatically receive a three-year sentence, adding another 5,000 prisoners over 10 years.

Automatic sentences for serious repeat offenders and drug traffickers will also push up figures regardless of sentencing policies by the courts.

The report also notes that Home Detention Curfew, in which inmates serve the latter part of their sentences in the community but tagged electronically, has not led to as many people being released early as expected.

Projections before the scheme was launched a year ago estimated it would lead to a 4,000 drop in the prison population. That has been revised to 2,000.

Tagging may improve

However, the Prison Service, which considers the scheme a success, said it was still bedding in and numbers could rise further.

There are currently 67,800 people in prison in the UK - roughly 0.1% of the population - but the number is expected to reach 70,000 by 2,002.

Three other new prisons are being planned along with extensions to existing establishments.

A Prison Service spokesman said the inmate numbers were constantly being monitored and the service could draw on emergency accommodation - such as prefab prison blocks or floating prison ships - if necessary.