Figures from the Office for National Statistics showed the Treasury’s coffers fell into the red last month, the first time they have done so in October in 10 years.
If the borrowing continues at the same rate, then the government deficit could reach double figures by the end of the financial year.
The news prompted speculation that government spending will have to be cut back or taxes raised to meet the deficit.
Brown has budgeted to borrow £33bn.
The Treasury though remained upbeat that they would meet their financial targets.
A spokesman said: ‘We are meeting our fiscal rules and will set out the new forecast in the Pre-Budget Review.’
This takes place on December 2.
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