The government was accused of "maxing out the nation's credit card" after figures showed that Britain's budget deficit ballooned to a record £20bn in November pushing the national debt to a post-war high.

In last week's pre-budget report Alistair Darling revised up his prediction for government borrowing this fiscal year to a record £178bn from the £175bn forecast in April's budget, or nearly 13% of national income. Of that total, £20bn was run up last month alone, although the number was smaller than the City had expected.

Shadow Chancellor George Osborne said: "In the run up to Christmas, Gordon Brown is maxing out on the nation's credit card – and doesn't care how we are going to repay these debts in the future. The record public borrowing figures equates to almost £1,000 for every family this month alone.

"Now we're all paying for Labour's failure to fix the roof when the sun was shining – and after last week's PBR, we know they have no credible plan to get this debt under control."

The public finances have been wrecked by the combination of the recession, which has pushed up spending on welfare as unemployment has surged and as the financial crisis has clobbered tax receipts from the City.

The Office for National Statistics also said that borrowing in the first eight months of 2009/10 had jumped to £106bn from £49bn in the same period of last fiscal year. It said spending was 6% higher over the period while receipts were 9% lower,

The national debt rose to £845bn last month, up from £706bn a year ago and equivalent to 60.2% of gross domestic product - way above the 40% level the government considered its ceiling before the recession came.

If the bank bailouts, which added some liabilities to the public sector balance sheet, are excluded, however, the national debt is closer to a more respectable 50% of GDP.

"These figures confirm that when the economy does move out of recession there is a massive task ahead to sort out the public finances," said Liberal Democrat Treasury spokesman, Vince Cable.

"It reinforces the lack of leadership given in the government's pre-budget report, which gave no indication of how the deficit will be reduced in a systematic way. But we need to be careful not to fall for Conservatives' remedies, which would involve sudden deep cuts in the public sector at a time when the economy is still very fragile."

The British Chambers of Commerce said the figures raised fresh questions over the country's "AAA" credit rating.

"The current situation poses serious threats to Britain's international credit rating, particularly if the government does not reinforce the inadequate measures announced in the Pre-Budget Report," said BCC chief economist David Kern.

"While it is difficult to implement large spending cuts until the recession is over, the government must be more explicit in its plans.

VAT is to rise on 1 January, to 17.5% from 15%, which will bring in some extra revenue, but Thursday's retail sales figures suggested consumer spending is weakening, which could crimp extra income for the Treasury. Conversely, say analysts, the better-than-expected unemployment data this week may save the government some money on jobseeker's allowance and other benefits.

John Hawksworth, head of macroeconomics at PricewaterhouseCoopers, said he thought the budget deficit would probably end the 2009/10 fiscal year higher than Darling is hoping, at £185 bn.

"Looking further ahead, we would still see a need for significant fiscal tightening after the next election (irrespective of the result) through a combination of tax rises and further spending cuts from 2011/12 onwards."

The Institute for Fiscal Studies, though, was slightly more optimistic about the current year. Gemma Tetlow, a senior research economist, said: "Extrapolating borrowing over the first eight months of this financial year, but adjusting for the estimated impact of policies which are expected to affect the relative timing of receipts and spending over the course of the year, suggests that borrowing for the year as a whole is almost exactly on course to meet the pre-budget report forecast published last week of £178 bn. [But] this would still be the highest level of borrowing since the Second World War."