British public borrowing rose to its highest last month for any September on record, showing Chancellor of the Exchequer Gordon Brown faces a tough battle to meet his fiscal target for the year.

Mr Brown, tipped to succeed Tony Blair as Prime Minister next year, aims to cut net borrowing to £36 billion this year from nearly 39 billion in the fiscal year to March 2006.

But after borrowing of £6.96 billion in September, the deficit in the first half of the fiscal year stood at £25 billion, some £4 billion higher than at the same time a year ago. Government expenditure was to blame.

"Some of this year's spending is front-loaded so the second half should be better though we still have some way to go to get the reduction in the deficit that the Chancellor is forecasting," said Geoffrey Dicks, economist at RBS.

Central government spending in the first six months of the fiscal year is 7.6 per cent higher than a year ago compared with Mr Brown's forecast of a 4.7 per cent rise for the year as a whole.

"This is affected by the timing of grants payments to local authorities and other factors," said John Hawksworth, head of macroeconomics at PriceWaterhouse Coopers.

"It remains possible, as we have seen in some previous years, that the Treasury will rein in all or most of this potential spending overshoot."

Receipts were also forecast to pick up strongly in the rest of the year as the economy has been expanding robustly.

But whatever this year's outturn, economists agreed spending will need to be restrained over the next few years to ensure Mr Brown's golden rule - which only allows borrowing for investment over the economic cycle - continues to be met.

Mr Brown has already pencilled in a sharp slowdown in expenditure growth in the years many expect him to be Prime Minister, saying he will keep public sector wage rises capped in line with the inflation target. That has also made it harder for the opposition Conservative party to promise tax cuts without also suggesting they may have to cut public services to fund them.

But a Conservative-sponsored report has suggested Britain should cut taxes to the tune of £21 billion, immediately raising speculation that is what the party's real plans were.