The prison population has been rising in the past few months

The Daily Mirror said a leaked document showed the go-ahead had already been given to new "superjails" in Garforth in Leeds and Barking in East London.

The Home Office would not comment on the claim, but a Prison Service source said: "Options are being considered."

Prison reform groups commented that new jails did not cut overcrowding, and numbers in jail should be cut.

These so-called super-size jails will simply increase the numbers of people leaving prison homeless, jobless and ready to offend again Juliet Lyon Prison Reform Trust

The Prison Service source said: "Options are being considered, including new build and purchasing sites.

"There is a lot of pressure on the prison population and a lot of the estate needs modernising."

He said the Home Secretary, Charles Clarke, would be presented with proposals on which sites should be used, but no decision had been reached yet.

The Mirror said the leaked document said the Barking jail was needed "urgently" to meet a shortfall of 7,000 places.

It said Garforth was the "best available site" in West Yorkshire, according to the paper.

But prison service sources said there was no decision on the two sites mentioned in The Mirror.

Capacity

The current prison capacity is around 77,000 and is due to rise to just over 80,000 in three years, the Prison Service has said.

There had been an increase of 16,000 in the number of prison places since 1997. In March, a Howard League for Penal Reform report claimed that Britain's prisons were "bursting at the seams".

It said 76 of the 139 prisons in England and Wales were overcrowded in January.

Juliet Lyon, director of the Prison Reform Trust criticised the plan for "jumbo jails".

She said: "The appalling reconviction rate should show us that far from cutting crime, these so-called super-size jails will simply increase the numbers of people leaving prison homeless, jobless and ready to offend again.

"Instead of building jumbo jails, the Government should stick to its plans to reserve prison for serious and violent offenders and save costly places by investing in drug and mental health treatment and effective community penalties."

Frances Crook, director of the Howard League, said: "The public must choose whether to have nine new district hospitals or 100 schools, which equals one new prison. You can have one prisoner or two nurses."

She went on: "It's a great myth that prisons prevent crime because they don't. They hold people for a short period and when they get out they create more crime."