The amount of new public sector borrowing by Britain reached a record £15.9 billion for August.
The figure was larger than expected after higher inflation led to a rise in interest payments on index-linked government bonds.
However, the Office for National Statistics said receipts from taxes were still rising.
The BBC reports the latest figure means borrowing in the first five months of the financial year reached £58.1 billion.
However, the forecast for borrowing for 2010-11 as a whole remains £149 billion, down from last year's total of £155 billion.
A spokesman for the Treasury said the figure underlined the need for the government's forthcoming spending cuts.
"Today's borrowing figures demonstrate just why the government needs to tackle the deficit," he said.
"If the government had not announced decisive action to bring borrowing down, debt interest would have been over £65 billion by 2014-15, more than is spent on schools or defence."