Britain's borrowing jumped to a higher-than-expected £14.2 billion in November, putting pressure on Chancellor George Osborne to meet his targets to bolster the nation's finances.
The Office for National Statistics (ONS) said public sector net borrowing excluding state-backed banks leapt £1.3 billion year-on-year in November, taking the overall figure so far this financial year to £66.9 billion.
The November figure is higher than the £11.1 billion expected by economists.
While year-to-date borrowing is £6.6 billion less than a year earlier, experts warned Mr Osborne will struggle to meet his £73.5 billion target for the full year.
James Knightley, economist at ING, said although the monthly data can be volatile, "barring a dramatic improvement in the trend, it is looking likely to be missed by possibly more than £5 billion".
Mr Osborne's Autumn Statement showed that independent forecasts from the Office for Budget Responsibility (OBR) confirmed the UK would move into surplus by 2020, while also confounding experts by revealing public sector net borrowing was set to narrowly beat this year's target.
The OBR also handed the Chancellor an early Christmas present when it unveiled a £27 billion boost to the public finances over the course of the parliament.
But experts said the improvement this year is unlikely to match OBR predictions.
David Kern, chief economist at the British Chambers of Commerce (BCC), said: "The public finances are likely to be better this year than in the previous financial year, but the improvement may not be as large as the OBR suggested in the Autumn Statement.
"The underlying message remains that our budget deficit is still too high, and greater efforts are needed, through reducing current public spending and generating sufficient tax receipts."