Britain slumped into the red by a higher-than-expected £23.3 billion in November - the highest monthly borrowing figure since records began, official figures have revealed.
The figure, which excludes financial interventions by the Government, was a marked increase on the £17.4 billion a year earlier and beat the previous highest monthly borrowing record of £21.1 billion in December 2009, according to the Office for National Statistics (ONS).
Total public borrowing for the year to date now stands at £104.4 billion, the ONS said, creeping closer to the Government's target of £149 billion for the financial year.
Economists have warned the coalition is in danger of exceeding the target - and overshooting the Office for Budget Responsibility's recently downgraded forecast of £148.5 billion for the year.
The bigger-than-expected figure will be seen by Chancellor George Osborne as supporting the need for recent austerity measures, which include an £81 billion package of spending cuts and a hike in VAT next year.
Economists were braced for a rise in the year-on-year level of public borrowing in November but none predicted a figure so high.
Jonathan Loynes, chief economist at Capital Economics, said: "Given that the economy has expanded rather more quickly than anticipated over recent quarters, we might have expected somewhat lower current borrowing, even allowing for the usual lags."
The unprecedented £5.9 billion leap in borrowing was mainly due to Government spending - up 10.8% on last year. EU contributions and spending on health and defence were particularly high last month, the ONS said, while VAT receipts dipped by 0.1%.
Net debt is now £863.1 billion, which represents 58% of gross domestic product (GDP) - another monthly record.
A spokesman for the Treasury said the latest figures backed the Government's fiscal-tightening measures and were in line with the forecasts of the tax and spending watchdog. He said: "November's borrowing figures show why the Government has had to take decisive action to take Britain out of the financial danger zone. These outturns are also in line with the Office of Budget Responsibility's latest forecast for borrowing to fall by almost £10 billion this year compared to last, and for tax receipts to increase by over 7% year on year."