The £2.7 billion spent on road maintenance and repairs each year has until now been classified as normal spending. But it will shortly be reclassified as “investment” after a decision by the Office for National Statistics (ONS).

Since the Chancellor is allowed to borrow to pay for items classified as “investment”, but not spending, this means that he is more likely to keep within his strict borrowing rules without having to raise tax by letting debt take the strain.

The Conservatives said that Mr Brown was cooking the books. “This is a classic case of fiddling the figures. The Chancellor has a problem with his forecasts of public borrowing. Solution: reclassify spending,” Oliver Letwin, the Shadow Chancellor, said. “If ever there was proof that we need an independent national statistics office this is it.”

The ONS denied that it had been leant on by Mr Brown or the Treasury to make any changes to how it calculates its statistics, saying that the revisions had been decided on as part of the normal review of methodology.

“There was absolutely no improper influence. There was a joint study with Treasury statisticians but the decision to make the revision was made by the National Statistician alone, and its implementation fully accords with the National Statistics Code of Practice,” a spokesman said.

The ONS will give details of how much of the £2.7 billion budget will be reclassified as investment at the end of the month, but said yesterday that it would have a “substantial” effect on the data.

Even if only part of the roads budget is reclassified as spending it would help Mr Brown to reassure voters that taxes will not have to rise after the election to pay for increased spending on schools and hospitals.

City economists said that every little helped in balancing the books. “On its own projections the Government will meet its own fiscal rules. Others are more doubtful, and this change in the accounting will give Gordon Brown some extra leeway in meeting his ‘golden rule’,” said Ben Broadbent, an economist at Goldman Sachs.

Other economists questioned whether the change in how road repairs are classified was the first of many other revisions that would put a more favourable gloss on the public finances.

“I would like some reassurance that we are not going to see lots of items reclassified as investment to help the figures look better,” a City analyst said.

Details of the proposed changes were published yesterday in background notes to new public finance data. The figures contained more good news for Mr Brown. A surge in tax revenues from companies and the public last month resulted in the biggest monthly surplus for three years.

Corporation tax receipts rose by 30.6 per cent compared with January last year, and income tax receipts were up 19.1 per cent compared with last January. That will give Mr Brown more room to manoeuvre as he plans his Budget, due to be delivered next month. Some forecasters have been predicting tax rises of up to £10 billion after the election to fill what they describe as “a black hole”.

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