Economic growth over the rest of this year would have to accelerate to a rate not seen since the boom of 2000 if Gordon Brown is to meet his forecast for 2005.

In recent weeks the chancellor has prided himself on the accuracy of his economic forecasts, but Mr Brown has pencilled in an economic expansion of 3 to 3.5 per cent for 2005, well above the consensus of independent economists.

Any growth shortfall would damage the chancellor's ambition to cut the budget deficit, increasing the risk that the government will fail to meet its self-imposed rule only to borrow for capital investment over the economic cycle.

The Office for National Statistics on Friday reported economic growth of 0.6 per cent in the first quarter of this year, a figure broadly in line with independent economists' expectations.

That being the case, either Friday's figure must be revised sharply higher or the economy must accelerate over the rest of 2005 if Mr Brown is to hit his Budget forecast.

Economic growth must average 0.9 per cent for the remaining three quarters of the year, something that has not been achieved in any three-quarter period since the stock market and economic boom of 2000.

Ed Balls, former chief economic adviser to the Treasury and now a Labour candidate, said quarterly growth figures were often revised higher. “It would be a very brave forecaster indeed to predict on the basis of the first estimate of the first quarter of the year that the growth forecast was going to be inaccurate,” he said.

However, only two of the 41 economists surveyed by the Treasury in April think the chancellor will meet his forecast. The consensus is that the economy will grow by 2.5 per cent this year.

With consumer spending slowing, oil prices rising and increased concerns about global economic growth, Jonathan Loynes of Capital Economics said “it is very likely that growth will come in much weaker”.

Peter Spencer of the Ernst & Young ITEM Club said: “The Treasury needs everything to go right. That means growth of at least 3 per cent and the tax burden has to rise at the stupendous rate in the forecast.”

Mr Brown has attacked any suggestion that the UK economy might not perform as well as he has predicted.

Speaking at an International Monetary Fund news conference last weekend, he said: “I say this with respect to the staff of the IMF, they have been wrong before about British growth . . . and I believe the figures are wrong again from the IMF.”

Martin Weale, director of the National Institute of Economic and Social Research, said that although it was possible Mr Brown might hit his forecast “the fact that the Treasury was spot on last year does not mean it will be again this year.” He added: “Most forecasters are aware of the risks about boasting about forecasting records.”

On Saturday, the chancellor set out his finance pledges for health and education in some of the world’s poorest countries. Speaking in Scotland, Mr Brown said that the issue was an “urgent moral priority for the Labour Party” and if re-elected he would create an international financing operation to tackle it.