Data released by the Office for National Statistics two days before tomorrow's Budget show the Government has borrowed pounds 31.7bn so far during the financial year. That is pounds 500m more than at the same stage last year.
Mr Brown has set an annual borrowing target of pounds 37bn.
Douglas McWilliams, chief executive of the Centre for Economics and Business Research , said the Chancellor was facing difficulties on two fronts.
He said: "First, various areas of public expenditure look under-funded, despite recent large increases.
"These include the NHS and pensions and he may have to make provisions for these which go beyond his contingency allowances."
The Chancellor also needed rapid economic growth to bring borrowing back on balance.
The Treasury said public finances were broadly on track to meet its full year forecasts.