By Jon Sindreu and Nick Winning 

LONDON--The British government borrowed more than expected in September, increasing the risk that it will overshoot budget-deficit targets it set as part of its sweeping government-spending cuts.

The U.K. government borrowed GBP11.8 billion ($19 billion) for the month, excluding public-sector banks, up from GBP10.3 billion in September 2013, according to official figures released Tuesday by the Office for National Statistics. Many economists expected borrowing needs to decrease slightly.

The rise in borrowing is a headache for Treasury chief George Osborne who has promised to eliminate the U.K.'s government deficit by the fiscal year ended in March 2019. To do that, total public borrowing would need to be GBP96 billion for the 12-month period to March 2015, estimates the Office for Budget Responsibility--the U.K.'s independent fiscal watchdog. But six months into the fiscal year, borrowing is already 60% of that level.

Mr. Osborne, who is Chancellor of the Exchequer, and his close ally Prime Minister David Cameron took office in 2010 promising to wrestle down the U.K.'s budget deficit by reigning in government spending. Their credibility in handling the economy will be a key issue in the next general election, scheduled for May.

The borrowing figures are "disappointing," said Rowena Crawford, an economist at the Institute for Fiscal Studies, a London-based think tank. Ms. Crawford, like many other economists, anticipates that borrowing needs will decrease in the coming months thanks to some expected deferred income tax and a forecast increase in wages. While Mr. Osborne still has time to reign in borrowing as the election nears, he may have less scope for maneuver, some economists say.

One issue for Mr. Osborne has been lower-than-expected revenues from income taxes, which have failed to pick up as many workers have yet to see an increase in their salaries despite the economy picking up.

Much of the increase in borrowing in September was due to central government departments, which saw spending increase by 5.4% over the prior-year period while revenues only rose by 3.1%.

A Treasury spokesman, in a statement Tuesday, attributed the increased borrowing to the continuing impact of the recession and said the U.K. was vulnerable to the problems being experienced by economies in Europe and elsewhere. The spokesman said the U.K. was growing faster than other large economies, adding jobs, and the government had reduced the deficit by more than a third since it took office. He said these were signs that "the government's long-term economic plan is working."

Economists expect the U.K. economy to show continued growth when gross domestic product figures are reported Friday, albeit at likely a slower rate than the 3.2% increase in GDP in the second quarter, over a year earlier.

The U.K. government's debt is 79.9% of GDP, excluding public sector banks--slightly above what was forecast in the March budget for the whole fiscal year. Four years ago, in September 2010, the figure was below 54% of GDP.

"These figures are a serious blow to George Osborne," said Chris Leslie, a spokesman on the economy for the main opposition Labour Party. "Not only is he set to break his promise to balance the books by next year, but borrowing in the first half of this year is now 10% higher than the same period last year."

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