The cost of building three superjails is not included in the £1.2 billion budgeted for a prison-building programme, it has emerged.

Ministers have promised up to three jails housing 2,500 prisoners each as part of a drive to produce 10,500 extra spaces to cope with the rising number of offenders.

But the Ministry of Justice has admitted that an extra £1.2 billion announced by Jack Straw for delivering an extended prison-building programme does not actually include building the three “Titan” jails.

A spokesman said: “The £1.2 billion will cover the capital and running costs of additional places coming on stream during the next spending review period (2008-09 to 2010-11) as well as additional funding for offenders to be managed in the community."

Asked whether the money included building the three jails, the spokesman said in a statement: “The £1.2 billion will enable us to progress with developing Titan prisons.”

It is understood among prison officials that only money to find and acquire sites for the three jails is included in the £1.2 billion. The rest of the cash is to be spent on providing other accommodation, including converting RAF Coltishall in Norfolk into a Category C closed jail, looking for two prison ships, bringing forward existing jail modernisation schemes and paying for monitoring more offenders in the community.

Mr Straw, the Justice Secretary, will face questions today from the Commons Justice Select Committee over how the Government is to fund the new 10,500-place building programme on top of an existing pledge to provide 9,500 spaces by 2011. The Titan jails were the centrepiece of a review of prison capacity conducted by Lord Carter of Coles, a friend of Mr Straw. Lord Carter recommended that one superjail be erected by 2012, with a further two by 2014, although Mr Straw announced only that “up to three large Titan prisons” would be built – casting doubt on whether three would be built given the costs involved.

Lord Carter’s report said that it would take up to five years to procure, design and construct a superjail. The size of the complex, which could include five separate units behind one perimeter fence, would enable savings through centralised support services such as catering, medical and administration. The report also recommended using offsite prefabricated units to save money. At present the private security industry works on the basis that one prison space in a secure Category C closed prison costs about £150,000.

Andrew Tyrie, a Conservative MP on the committee, said: “It looks very much as if Jack Straw has misled the public about the real cost of building these prisons. He now needs to explain how much Lord Carter’s £1.2 billion will buy and therefore how big a black hole will still be faced by the Ministry of Justice’s budget.”

He added that the costing for the Titan prisons appeared to have been “written on the back of an envelope”.

Juliet Lyon, the director of the Prison Reform Trust, said last night: “We have two fears – the first that, whatever the detail, the money needed to run 10,500 more prison places will be a severe drain on scarce resources for years to come; and the second that, at a fraction of the cost, sensible crime-cutting measures, such as Baroness Corston’s reforms of the women’s justice system or residential treatment for addicts, could have been introduced without further delay.”

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