The Conservatives want to build more prisons so offenders can be kept behind bars for longer.
The Tories have called for more prisons to be built
They are considering allocating £15 billion to create more places in Britain's near-capacity prisons.
The plans, revealed by the Sunday Telegraph, would be funded by scrapping government plans for identity cards.
The Chief Inspector of Prisons has also warned prisons are close to putting up "house full" notices with less than 1,800 places left.
The Conservative plans follow a row over the early release of offenders after it emerged last week that serial paedophile Craig Sweeney could be free in five years.
Tory leader David Cameron wants criminals to serve greater proportions of their term behind bars and is looking at ways to fund additional prison space.
Options include using the money saved from ditching ID cards, due to be introduced from 2008.
'Decent and humane'
Shadow home secretary David Davis told BBC's Sunday AM programme he was "quite sure" that more prisons are needed.
"At the moment, if you're a prisoner and let's say you can't read, you haven't got any skills, you're addicted to crack cocaine, to sort out any of those problems takes time in prison, takes courses.
"If you're being transferred after three or four weeks to another prison, then to another you'll never complete that.
"So actually, to do the job of a decent and humane as well as effective prison system, you need more prisons."
Mr Davis also said he did not know how many more prisons were needed.
But a party spokeswoman stressed that no spending commitments had been made over this issue.
She said: "This is certainly something that we are looking at. We definitely recognise that prison places need to be expanded.
"One option for funding that would be re-directing money that would be spent on ID cards.
"But there are no spending commitments at this time. They will be made closer to the next general election."
The Chief Inspector of Prisons, Anne Owers, told The Sunday Times that with fewer than 1,800 of 79,500 prison places left, jails were close to capacity.
"We are looking at a system where prison numbers are rising inside what is already a hugely pressured area," she said.
"That may hit the buffers soon at a point where there are not any more spaces.
"I think the point will come at which prisons have to put up 'house full' notices."
Commons leader Jack Straw, also speaking on Sunday AM, said "We have built more prisons and prison places and the chances are that more have to be provided.
"I have never had a problem about building more prisons."
Labour Party chairman Hazel Blears challenged the Conservative Party to publish their £15bn prison-building plan.
"I want the Conservatives to set out their plans on prisons. If they do not the public will conclude that there is no more substance to these Tory proposals than there was to earlier Cameron pronouncements on chocolate oranges," she said.
The prison population stood at 77,785 on Friday, leaving room for 1,715 more criminals.
But there have been 148 more inmates a week since May, the paper said.
Juliet Lyon, director of the Prison Reform Trust, said public money would be far better spent on community rehabilitation.