Senior ministers have hinted that the 50p top rate of income may only be temporary as they fend off accusations of the death of New Labour.

Gordon Brown and senior members of his Cabinet moved to resuscitate the New Labour legacy today by insisting the Budget announcement of a 50p top rate of tax - the highest level for 30 years - was not a political or ideological gesture.

Lord Mandelson, who opposed Gordon Brown's attempts to get a 50p rate introduced when Tony Blair was Prime Minister, instead insisted it was a practical necessity born of exceptional economic circumstances.

"It's most certainly not the end of New Labour. We are not a high tax party. We don't tax for its own sake," the Business Secretary said.

Mr Darling also toured broadcasters to stress the new top rate would only apply during exceptional circumstances.

Saying he had a "duty" to reduce borrowing, the Chancellor said: "I have also had to go to those people who have earned the most over the past few years, people on over £150,000, and say 'you are going to have to contribute while we resolve this situation'."

When Labour last suggested a 50p top rate of tax, during the early nineties, it was proposed as part of a permanent realignment of the tax system designed to redistribute wealth.

Earlier, Mr Brown stressed that the tax rise for high earners in yesterday’s Budget was “not taxation for its own sake... it’s tax for a purpose”.

Meanwhile the Tories said that the Chancellor has indulged in “complete fantasy” rather than using his Budget to provide a plan for tackling the true scale of the recession.

George Osborne said it was a “tragedy” that Mr Darling had not been honest about the problems facing Britain’s economy or taken responsibility for mistakes that had led to it.

He told MPs that the central task of yesterday’s Budget statement should have been to provide confidence in the future but that Mr Darling had failed to do so.

Mr Osborne was particularly scathing about the Chancellor’s growth predictions, which envisage 1.25% of expansion for 2010 and 3.5% by 2011.

Economists and independent analysts had almost immediately rejected these as over-optimistic, Mr Osborne said, as had figures from the International Monetary Fund (IMF) released less than an hour after the Chancellor sat down.

Mr Osborne said: “We have this extraordinary situation where the Chancellor of the Exchequer stood there yesterday, announced the worst public finances ever heard in the House of Commons; told us we would be borrowing more in the next two years than all previous Chancellors have announced at that despatch box combined; said he would double the national debt, and yet he was guilty of being too optimistic.

“That is the scale of the mess this Government has created. And the tragedy of yesterday was that instead of being honest about that mess, instead of taking responsibility for the mistakes that have been made and giving us a credible plan to pull Britain through, we got that complete fantasy.”

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