Conservatives have accused Gordon Brown of planning a secret "tax bombshell" after it emerged the Treasury had thought about increasing VAT by 1% to 18.5% after a General Election.

A document accidentally published on the internet stated the increase would come in 2011 to help pay for a 13-month reduction in the main VAT rate from 17.5% to 15%, which will begin on December 1.

The Treasury said the draft background note was drawn up last week ahead of Monday's Pre-Budget Report (PBR) but should never have been seen.

A spokesman said Chancellor Alistair Darling considered the proposal but rejected it, insisting there are no plans for an increase in VAT.

But David Cameron said the paper was proof that Labour plans to hike taxes following an election expected in 2010, claiming it would cost every family about £200 a year.

The row blew up ahead of a House of Commons debate on the dramatic measures announced on Monday, including a £20 billion fiscal stimulus to restore growth, a new 45% top rate of income tax for high earners and a 0.5% increase in National Insurance in 2011.

Tories claimed there is a £10 billion "black hole" in the Government's finances for 2011-12, as set out in the PBR.

Mr Cameron suggested the Prime Minister and Chancellor may be secretly planning to use a VAT hike to fill it.

He said: "The Prime Minister specifically said, 'No hidden manifesto, everything above board'.

"And yet now we know they are planning a secret tax bombshell, a VAT hike that would hit every single family in the country after the election. That's why the budget didn't add up."

But treasury minister Angela Eagle dismissed his claim, saying: "It is a pure administrative error on the part of HM Revenue and Customs.

"For anyone to think it is anything else is complete scaremongering and opportunism of the worst sort."

The document detailed how the rate would fall to 15% for 13 months before returning to 17.5% on January 1 2010. But unlike the Chancellor's statement, it went on to say the rate would "subsequently increase to 18.5% in 2011-12."

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