By Jon Sindreu And Nick Winning 

LONDON--The British public sector borrowed more than expected in September, increasing the risk that the government will overshoot its austerity targets.

According to figures released Tuesday by the Office for National Statistics, public bodies in the U.K.--excluding public sector banks--borrowed GBP11.8 billion ($19 billion), up from GBP10.3 billion in September 2013. This is above forecasts by economists polled by The Wall Street Journal--who expected borrowing needs to go down--and leaves the budget deficit at 4.2% of gross domestic product.

Central government bears most of the responsibility in the increase of the budget deficit: Compared with the same period in the 2013-14 fiscal year, it has increased expenditure by 5.4%, while receipts only rose by 3.1%.

Since the financial year started in April, the Treasury has mainly struggled with low income-tax revenues. The reason is that salaries in the U.K. remain stagnant: Average weekly earnings grew by only 0.7% in the three months to August, official figures showed last week, while annual headline inflation was more than double the rate during the period.

As the British public debt is already above 125% of gross domestic product the Treasury is counting on the strong economy to boost tax receipts and curtail the budget deficit. This would allow Chancellor of the Exchequer George Osborne to keep his pledge of achieving a surplus by the 2018-19 financial year without further significant cuts in spending. Low income-tax revenues, however, threaten these plans.

For austerity to remain on track, the Office for Budget Responsibility--the U.K.'s independent fiscal watchdog-estimates total public borrowing would need to be GBP96 billion by March 2015. Half way into this period, borrowing is already 60% of that figure.

Although Mr. Osborne still has time to narrow the shortfall, the general election is right around the corner--May next year--and both the budget deficit and the declining living standards are major issues in the campaign.

Write to Jon Sindreu at jon.sindreu@wsj.com and Nick Winning at nick.winning@wsj.com