UK public finances suffered their worst June on record, figures from the Office for National Statistics (ONS) show.
Mr Brown may break his promise of meeting borrowing targets
Government borrowing rose to £7.3bn in June from £6.2bn during the same month last year - significantly higher than analysts' forecasts of £6.5bn.
Meanwhile, the current budget deficit stood at £6.4bn, £1.7bn higher than at the same time last year.
Experts said high spending was to blame, as grants to local authorities had been paid earlier than usual.
Elsewhere, economic activity appeared to be fairly healthy, with consumer spending picking up and tax receipts rising 5.4% year-on-year.
In his recent Budget, Chancellor Gordon Brown insisted he would meet his target of cutting the current budget deficit to £7bn and public sector net borrowing to £36bn.
That promise may now prove difficult to keep, as current borrowing for the year now stands at £16.4bn.
However, a Treasury spokesman insisted it was meeting its strict fiscal rules and would "continue to do so".
Global Insight chief economist Howard Archer warned that in order to meet his targets, Mr Brown would need "the help of healthy growth over the coming months, as well as increased restraint in government spending".
Douglas McWilliams, chief executive of the Centre for Economics and Business Research added that as well as tighter control of public spending, the Chancellor might also look to raise tax revenues.