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Economists have predicted Chancellor Alistair Darling's first Budget will include an admission that GDP growth may dip below 2% this year.

Mr Darling's pre-Budget statement in October cut the GDP forecast to between 2% and 2.5% - compared to 2.5% to 3% estimated by former chancellor Gordon Brown in March. But this could be revised for a second time, to 1.75% and 2.25%, amid slower consumer spending and continued fall-out from the credit crunch.

Such a downgrade will place further strain on the public finances. Barclays Capital estimates that up to £5 billion could be added to government borrowing by 2010/11 if the same GDP forecast range is also used for 2009.

While stronger than expected tax receipts in January will leave this year's public borrowing in line with Government forecasts at £38 billion, economists are expecting a sharp deterioration in the next two financial years.

Philip Shaw, chief economist at Investec Securities, said: "The Budget is likely to include downward revisions to GDP forecasts for this year and next.

"The gloomier outlook suggests that the Treasury's 2008/09 and 2009/10 borrowing forecasts will be raised significantly - we estimate to £39 billion and £41 billion."

However, he said it was likely that public debt figures would not incorporate a 'Northern Rock effect' in time for the Budget.

The move for the nationalised bank is expected to add £100 billion to the country's public sector debt, threatening to put the Treasury in breach of its golden borrowing rule.

Net debt in January stood at £512.4 billion, equivalent to 35.9% of GDP - up from 35.5% in January 2007. The Government's sustainable investment rule dictates that debt should not be more than 40% of GDP.

The ONS indicated in early February that Northern Rock's impact could add around 7% to the key measure, which would leave the benchmark well above the Government target.