Chancellor Gordon Brown's economic forecasts are far too optimistic, a number of independent experts have warned.
They believe that, during the next two years, there will be a shortfall of tax revenue and spending cuts or higher taxes will be necessary as a result.
In Wednesday's Budget speech, the chancellor lowered his economic forecast for the second time in just six months.
But Mr Brown said there would be a strong recovery from 2004 onwards - a claim that independent analysts dispute.
Gordon, the optimist
"Gordon Brown is in denial," says Ernst & Young's Item Club, a forecasting group that uses the Treasury's model of the UK economy.
The Item Club argues that Mr Brown should have flagged up that taxes will have to go up from next year to rein in the imminent budget deficit.
This year's twice-reduced official growth target now ranges from 2% to 2.5%.
That is close to the average of independent forecasts of 2.4%, but the Item Club's economists doubt that even that figure will be met.
Others, for example the analysts at HSBC bank, are even more pessimistic, warning that the UK economy could grow by as little as 1.5% this year.
But all agree the real crunch time will come in 2004 and 2005.
'Hope for the best'
To balance his books, the chancellor has pencilled in a growth rate of between 3% and 3.5%.
But many experts - from the Institute for Fiscal Studies (IFS) to City analysts to the Item Club - agree that this would be a very-best-case scenario.
At the IFS they call Mr Brown's numbers a "hope for the best Budget" that "continues to expect the economy to bounce back and the public finances to bounce back with it".
But they doubt his figures, saying they add up only if the economy performs, stock markets soar again and big City banks make bumper profits and pay hefty taxes.
Tax revenue from huge City pay packets is another key element in Treasury calculations.
The IFS experts worry that it is unlikely that all three revenue streams will start gushing again and so predict a multi-billion pound funding shortfall.
The Item Club agrees, saying it would be "some miracle" for the economy to grow that fast, and even then "it would not generate enough tax revenue to cover the rising expenditure bills".
However, HSBC's experts caution fellow analysts that the chancellor pulled off such a stunt before - in 1998, when shamefaced City pundits had to acknowledge that Mr Brown's optimistic growth and revenue predictions came in right on target.
Meeting the golden rule
The chancellor's benchmark is his "golden rule", which states that it is acceptable to borrow to invest as long as any additional debt is repaid over the economic cycle.
The current economic cycle is expected to end in spring 2006, and Mr Brown predicts that he will emerge with a £32bn surplus.
The IFS agrees, but warns that the Treasury will find it difficult to reverse the setbacks it will suffer in the lean years that will follow.
And if the chancellor has got his numbers for the next three years wrong, they say, the economic and fiscal shock will be even more abrupt and have a harsher impact than a Labour government hoping for re-election would prefer.