Britain's public finances posted a record August deficit after inflation pushed up debt interest payments.

While analysts found the figures disappointing, they also noted that receipts continue to improve and the government is still on track to achieve its forecast for this year. The Treasury now estimates it received £3.5bn from its tax on bankers' bonuses, rather than the £2.5bn reported in the June budget. The net take, taking account of the impact on income tax and national insurance, is £2.3bn.

Public sector net borrowing came in at £15.9bn last month, official figures showed this morning, a record high for August and above City forecasts. The figure compares with the £14.1bn of borrowing seen in August 2009 and excludes the impact of the banking bailout.

Interest payments on government bonds, known as gilts, jumped to £3.8bn from £1.3bn as a result of higher inflation.

"Although broadly in line with the Office for Budget Responsibility's budget forecast, today's borrowing figures demonstrate just why the government needs to tackle the deficit," said a Treasury spokesman. "If the government had not announced decisive action to bring borrowing down, debt interest would have been over £65bn by 2014-15, more than is spent on schools or defence."

He noted that tax receipts for August, up 6.3% on last year, are broadly in line with the forecast for the whole year (6.6%) and receipts for the financial year so far, up 9%, are running ahead of the full year prediction.

"At first glance the numbers are disappointing," said Philip Shaw, chief economist at Investec. "Borrowing has risen on the back of strong spending and relatively weak tax inflows. However, there are some positive revisions to back data, so overall it does not look like the public finances will be blown off course for the year as a whole."

In the financial year so far, since April, the government has borrowed £58.1bn, down from the £61.9bn borrowing recorded this time last year. However, Britain's net debt is 56.3% of GDP, compared with 47.7% a year ago.

James Knightley at ING said: "Total receipts continue to improve, led by higher VAT due to the return of the 17.5% rate and with VAT moving to 20% [in January] the government's aim for the 2010/11 deficit still looks achievable."

Samuel Tombs at Capital Economics was more pessimistic. "August's overshoot reinforced our view that the government will struggle to achieve the ambitious fiscal tightening set out in the June budget and may therefore need to resort to further tax hikes or additional spending cuts in the not too distant future."