Optimistic Budget forecasts could leave a £10 billion black hole in government accounts, Gordon Brown has been warned.
A report from the Ernst and Young ITEM Club said that the performance of the UK economy in 2003 and 2004 would not meet the chancellor's expectations.
Limited growth could leave the government needing to raise taxes in order to pay for its spending increases, the economists warned.
The £10 billion funding gap is equivalent to £170 per head of the UK population and could add up to an extra three pence on the basic rate of income tax.
The Budget forecast for economic growth in 2003 was between two and 2.5 per cent, rising to three to 3.5 per cent in the following year.
But the ITEM Club suggested that output would grow by just 1.9 per cent this year and 2.6 per cent in 2004.
"Our analysis now suggests that the Treasury will need to raise about £10 billion in additional tax revenues to cover this shortfall," said Professor Peter Spencer, the ITEM Club's economic adviser.
The Conservatives said the report was a further blow to Gordon Brown's reputation.
"Not even three weeks has passed since the Budget and yet another highly respected economic think tank has cast doubt on the chancellor's economic forecasts, and by implication his reputation. Both have already been downgraded twice," said Michael Howard.
"The chancellor has misrepresented Britain's economic prospects in the past and he is now once again being less cautious than independent experts.
"When people hear experts talking about a £10 billion black hole, they will fear that it is a sword of Damocles hanging over the heads of British taxpayers and business."
Dods Parliamentary Communications Ltd