Government borrowing rose to £11.8bn in September, an increase of £1.6bn compared with a year earlier, according to the Office for National Statistics.
Economists were forecasting that borrowing would hold steady.
The latest figures are a set back for Chancellor George Osborne, who in March pledged to cut the budget deficit by more than 10% over the next 12 months.
Between April and September borrowing was £58bn, a rise of £5.4bn compared with the same period last year.
That is an increase of 10.3%. Last month’s figures showed borrowing between April and August was 6% higher than a year earlier.