A new "Titan" prison holding 2,500 inmates is to be built in the West Midlands as part of a £1.2 billion attempt to end the prison overcrowding crisis, ministers have revealed.
But the Government yesterday said it had not decided where the prison will go. One local MP last night urged ministers to reveal full details of the proposals as quickly as possible.
Caroline Spelman (Con Meriden) said: "There is no doubt that more prison places are needed, but we need to know where this major new prison is likely to be built."
Justice Secretary Jack Straw said he had secured an extra £1.2 billion for a building programme which will see the capacity of jails in England and Wales increase to 96,000 by 2014.
This will include three "Titan" jails, far bigger than any jail currently used in Britain, in the West Midlands, North-west and London.
The regions were chosen because they currently have the greatest shortage of prison places.
A spokeswoman for the Ministry of Justice said: "No specific sites have been identified so far.
"However, the intention is to use brownfield sites with good transport links."
One of the massive Titan jails will be in service by 2012, with a further two expected to be built by 2014.
The proposal was put forward by Government troubleshooter Lord Carter of Coles, who was asked by ministers to look at the way the prison system works from top to bottom.
His report recommended ministers should use an existing process to win planning permission for the massive jails as quickly as possible.
He pointed out officials can make an application directly to the Secretary of State in certain circumstances - a move which could avoid long-running opposition with people who attempt to block construction of a jail near their homes.
Lord Carter said it would be possible to build massive jails within four to five years, although he admitted it would be a "challenging" target.
There will be a shortage of prison places of up to 3,000 by next summer and up to 6,000 in 2009, his report said.
The problem would get worse if nothing was done, leading to a shortfall of 10,500 by the middle of 2014.
But Anne Owers, the Chief Inspector of Prisons, said smaller facilities were more likely to be effective.
She said: "All of our evidence, and the evidence collected from recent inspections, show that small prisons perform better.
"They are safe, they provide a more decent environment, they provide better engagement with staff. Larger prisons have not so far done that, and I am concerned about the notion that we build just for efficiency."
Harry Fletcher, assistant general secretary of probation union Napo, said: "The decision to build three large Titan prisons is extremely controversial.
"It goes against all previous advice, which has been for smaller units close to where prisoners live.
"It would be difficult to find sites, obtain planning permission, and manage and recruit staff for these super-prisons."
And Frances Crook, director of the Howard League for Penal Reform, described some of Lord Carter's proposals as "zany".
"You cannot build your way out of the failing behemoth that is our prison system," she said. "The proposal to build new Titan prisons will not solve the problem of spiralling prison numbers unless the Government accepts that its sentencing policies are unsustainable.
Mr Straw told the Commons the Ministry of Justice was also "actively looking" for a prison ship.