Alistair Darling came under more pressure last night to confirm that he considered raising VAT as high as 20 per cent as the row over the Pre-Budget Report grew.

George Osborne, the Shadow Chancellor, issued a demand under freedom of information laws for assessments of VAT increases by outside and internal experts to be published. A document setting out a proposal to raise the tax to 18.5per cent was mistakenly placed on an official website, prompting Conservative claims that the Government was planning a “secret tax bombshell”. Gordon Brown has refused to deny that still higher rates had been considered.

It emerged meanwhile that David Cameron is set to fight the next election on a pledge to reform public sector pensions. The ballooning cost to taxpayers of retirement benefits for public sector employees has forced the Conservatives to promise action. The annual public sector pension bill will reach £3.8billion by 2010, according to estimates in the Pre-Budget Report - three times last year's total of £1.2billion.

Until now the Tories have been wary of promising action to close the widening gap between benefits available to those retiring from the public sector and those payable to others. But Mr Cameron told a business forum in Manchester this week: “We have got to end the apartheid in pensions.” Hinting that he supported moves to less generous entitlements in the public sector, he said that his “vision, over time, is to move increasingly towards defined contribution rather than final salary schemes”.

Union leaders made clear last night that they would fight any proposals to change entitlements. The Liberal Democrats have accused the Tories of failing to tackle the issue head on. Vince Cable, the party's Treasury spokesman, speaking in an emergency Commons debate on the economy on Wednesday, said that the current system was out of control, and claimed that the Conservatives had failed to vote for an independent review last month.

Paul Kenny, leader of the GMB union, said: “David Cameron says that public sector pensions are too generous and should be done away with. This creates more clear blue water between the Tories and public sector workers. The 4.5million members in public sector pension schemes and their families should pay heed to Tory plans when it comes to voting at the next general election.”

Brendan Barber, the General Secretary of the TUC, said: “Public servants will be frightened and deserve to be told in much more detail what the Conservatives plan.”

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