Mr Darling cemented his reputation as the Chancellor with the worst forecasting record in modern history after he slashed his projection for British economic output this year to -3.5pc.

Unveiling the biggest increase in government borrowing since the Second World War, he conceded that Britain’s national debt will double to around £1.2 trillion in the coming years, and that the budget would not return to balance until 2018 or later.

In what economists described as a “fantasy Budget”, Mr Darling imposed swingeing new taxes on those who earn more than £150,000, which will raise as much as £5.5bn a year by 2012 – one of the biggest per-capita tax increases in recent history.

But while he will try to trim some departmental budgets, he will nevertheless increase the total amount the labour Government will spend this year and the next by £37.6bn. This indicates that if he does intend to balance the nation’s books, he will do so not with spending cuts but with tax rises.

However, the proposed tax and spending measures pale into insignificance against the scale and extent of the economic and fiscal crunch mapped out in the Budget small print.

The documentation reveals that this year’s economic contraction will be the worst in any year since the end of the Second World War. Indeed, according to the International Monetary Fund, this year is likely to be the worst since the 1930s, as countries around the world slump into a synchronised slowdown.

The Chancellor predicted that despite shrinking by 3.5pc this year, the UK economy would start growing again “towards the end of the year”, with 1.25pc growth next year and 3.5pc from 2011 onwards. Last night economic historians were trying in vain to find examples of developed countries that had stomached so significant a one-off drop in growth and yet managed to recover within 12 months.

As the IMF pointed out last week, recessions associated with financial crises tend to be more protracted and virulent than almost any other type, and that is precisely what the UK faces.

Indeed, alongside many City economists, the IMF expects the UK economy to shrink by a further 0.4pc next year – significantly below even the Chancellor’s revised forecasts.

Dig a little deeper into the Budget and it becomes clear that the Treasury expects nothing short of a full-scale consumer recovery – if not boom – in order to satisfy its projections. Such a prospect seems unrealistic if not outlandish, according to City experts.

Even taking at face value the Chancellor’s forecasts, the UK will have to borrow some £175bn this year and £173bn the next to make up for the shortfall in tax revenues and extra demands on the public purse from increased social welfare spending.

At over 12pc of gross domestic product, these represent the worst years for the public finances since the 1940s. However, the optimistic economic forecasts are doubly significant in this case because higher growth means a lower deficit. Should the Chancellor’s economic projections be proven wrong, the eventual outcome for the national accounts will be worse still, according to City analysts, with total borrowing likely to surpass £200bn and not to peak until next year.

The tidal wave of extra debt will push up Britain’s net national debt from below 40pc – one of the ceilings set by Gordon Brown in 1998 but since abandoned – to almost 80pc.

The Treasury said it still had no plans to reinstate any fiscal rules to bring its borrowing back under control in the future. In fact, it said it did not expect the budget to come back into balance until 2018 or beyond.

The Budget came amid continuing bad economic news, with the Office for National Statistics announcing that unemployment jumped by 177,000 to 2.1m in the three months to February, taking the jobless rate to 6.7pc.

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