An administrative error by government tax officials has led to a major row over the government’s plans for VAT.

Opposition parties have accused the chancellor of planning to increase VAT to 18.5 per cent after policy advice was inadvertently published on an official government website.

Alistair Darling announced in Monday’s pre-Budget report that he would be cutting VAT to 15 per cent for 13 months before returning it to its current level of 17.5 per cent.

But a document signed off by Treasury minister Stephen Timms and placed on the Office of Public Sector Information’s website by HM Revenue and Customs (HMRC) officials said VAT would “subsequently increase to 18.5 per cent in 2011-12”.

Conservative leader David Cameron said the document proved the government had a “hidden manifesto”.

“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,” he said.

Treasury minister Angela Eagle denied the claim, insisting that the publication of the document was “a pure administrative error on the part of HM Revenue and Customs”.

She added: “For anyone to think it is anything else is complete scaremongering and opportunism of the worst sort.”

It is not the first time the department has incorrectly released data - it was a year ago this month the chancellor was forced to tell MPs that HMRC had lost two discs containing information about 25 million child benefit claimants.

After the policy documents appeared on the web, the Treasury issued a statement saying that the increase to 18.5 per cent had been just one option presented to ministers.

“This was an option that was considered and rejected. In all Budgets, ministers consider a range of policy options, most of which are rejected,” a spokesman explained.