Much weaker than expected tax revenues meant that government borrowing last month set a September record of £4.8 billion, almost double the figure for the same month last year. The bleak September data was blamed by the Treasury on erratic factors affecting spending.

But the bad news from last month ended a highly disappointing first half to the financial year, during which the Government borrowed £22.75 billion — more than two thirds of the Chancellor’s £33 billion full-year forecast.

The figures left economists giving warning that unless tax revenues rebound sharply in the rest of the year, Mr Brown is on course to breach his “golden rule” for the public finances by borrowing at least £3 billion more than his Budget projection.

Analysts said that the threat of post-election tax increases was growing rapidly.

The Ernst & Young ITEM Club, which uses the Treasury’s own model to analyse the government finances, predicted that Mr Brown could exceed his borrowing forecast for 2004-05 by £6 billion.

The weakness of tax revenue was “very disappointing”, given the recent strength of the economy, which has matched Mr Brown’s hopes, ITEM’s economic adviser, Peter Spencer said.

Detailed figures showed that total taxes paid in the first half of the financial year have risen by 6.8 per cent from the same period in 2003-04, while the Chancellor is counting on a 7.8 per cent gain.

Income and capital gains tax receipts are up by 4.7 per cent so far, against the 7.4 per cent planned. Company tax payments have risen by 14.8 per cent, against a planned 21.3 per cent rise.

Professor Spencer and other analysts also emphasised a growing threat to Mr Brown’s golden rule, which says he will borrow only to invest.

This means that the Government’s “current” budget, for day-to-day spending, must stay in the black over the economic cycle.

But revisions in yesterday’s data showed that this has now sunk into the red, so that the Chancellor needs a fast turnaround in revenue to avoid breaking his rule.

Christine Frayne, of the Institute for Fiscal Studies, said that Mr Brown had planned for borrowing on the current budget to halve in 2004-05 year but there has been no improvement.

“If this trend continues, he is on course to break his golden rule,” she said.

Jonathan Loynes, of Capital Economics, predicted that the trend meant that “taxes are almost certainly set to rise” after an election due next year.

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