The Tories say they would end the early release scheme
The Tories have been accused of a U-turn on their pledge to build 5,000 new prison places in England and Wales.
The policy, announced by David Cameron in March 2008, was intended to allow a Tory government to end the automatic early release scheme.
The Tories now say they will create enough places to end early release but will not commit to the 5,000 figure.
Prisons minister Maria Eagle told the BBC: "This is a huge U-turn from David Cameron and their justice team."
"As recently as June this year [shadow justice secretary] Dominic Grieve was saying this was pledge number one."
She added: "There is nothing that has changed to make them suddenly change their mind. They have just realised, as we said last year, that their policy ... is not properly costed and it is incoherent."
Mr Cameron announced the scheme last year, saying older inner-city jails would be sold off to pay for more prisons on cheaper land elsewhere.
He wants to end automatic release for prisoners who have served half of their full term - a policy introduced in 2007 to ease overcrowding.
Under the current government guidelines, since June 2007 more than 60,000 prisoners have been released up to 18 days before the halfway point of their sentence.
The Conservatives say they are still committed to ending the early release scheme but have confirmed that they no longer plan to build all 5,000 new places.
The party said the sell-off of Victorian prisons had always been a long-term aim and acknowledged that the recession meant they would only fetch "rock bottom prices" at present.
Channel 4 News said the pledge had now been downgraded to "several thousand" places.
A Conservative Party spokesman accused Gordon Brown of having "choked" Home Office funding for prisons as chancellor and releasing "70,000 criminals from jail early" as prime minister.
He said: "A Conservative government will be committed to clearing up the mess this government leaves behind.
"That means building the prison places to end Labour's reckless early release scheme, and introducing a rehabilitation revolution to cut soaring reoffending rates."