The Conservatives are facing accusations of an £8 billion black hole in their NHS policy after government health officials calculated a massive shortfall in their pledge to double the number of single rooms in hospitals, The Times has learnt.

The Department of Health said yesterday that it estimates that the cost of building a further 45,000 single rooms – using standard NHS costing techniques – will be £9.51 billion, rather than the £1.57 billion estimated by the Tories.

The Conservatives rejected the Civil Service calculations, saying that they were “absolutely confident” of their original costings. The discrepancy could, however, prove an embarrassment both for Andrew Lansley, the Shadow Health Secretary, and George Osborne, the Shadow Chancellor, whose department signs off all Tory spending pledges.

Mr Lansley announced at the Tory conference last month that by the end of a first Tory term in office single rooms would be available for all expectant mothers, mental health patients and anyone undergoing an operation.

The Tories estimated that the cost of building rooms would be £35,000 a bed, made after the party undertook a Freedom of Information survey of NHS trusts that have carried out the work. But according to a written answer from Ann Keen, the Junior Health Minister, the NHS believes that the sum needed to provide a single room is £211,401 per bed.

If true, this will put fresh pressure on Conservative spending commitments and could result in savings having to be made elsewhere in the programme or a watering down of the commitment.

The Department of Health was asked to perform this calculation by Mark Hoban, a member of Mr Osborne’s Shadow Treasury team, in a written question.

Health Department officials suggested that equipment costs – each room costs £10,000 to kit out – and the need for a 15 per cent contingency fund partly explain the discrepancy. They also point out that fewer patients can be accommodated in single rooms – meaning that hospitals will have to find new space by converting wards in existing bathrooms and office areas.

They also suggest that even the £9.51 billion figure could prove to be an underestimate. Some wards may be too old to convert into single rooms, with some hospital sites making such a renovation unfeasible. The cost also does not include the cost of “decanting” patients elsewhere during the work.

Stephen Timms, Financial Secretary to the Treasury, said: “This latest misjudgment from George Osborne is another blow to his reputation as Shadow Chancellor.

“In the week after George Osborne admitted he made a mistake on Tory funding, will he now admit that his mistake on the NHS would leave an £8 billion black hole in the Conservatives' economic figures?”

A Tory spokesman said: “We are absolutely confident of our original costings. Our figures, obtained from Freedom of Information requests, are derived directly from the hospitals that have already done this.

“It is up to the Government to explain why its costings are so much higher, and, indeed, why a Health Minister was today unable to deny that her civil servants were ordered to change their figures in the light of our policy announcement.”

Department of Health guidance since 2001 has been that the proportion of single rooms provided in new hospital developments should be at least 50 per cent, and should in no case fall below 20 per cent.

New mental health accommodation should have 100 per cent single rooms. However, 82 per cent of new hospitals opened since 2001 do not have even 50 per cent single rooms.

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