A Labour insider has predicted that Tony Blair could break his pledge not to raise income tax rates without alienating voters.

In an interview with ePolitix.com, Matthew Taylor, the director of the IPPR, says that a second-term Labour government does not have to stick rigidly to all its pledges.

He said Blair should be wary of any policy which will box him in after the election.

He told ePolitix.com: "I think that generally one should tie one's hands as little as possible in these areas. And I think all parties recognise that if the economy was to suddenly change direction that all bets are off."

Labour's pledges were based upon the economy growing as predicted, he said.

"These are promises that that assume the economy remains relatively stable. You would be a very irresponsible government if you did not respond to a massive burst of growth or much more likely a recessionary position because you had made a manifesto commitment two and a half years before," said Taylor.

He also welcomed Oliver Letwin's intervention in the tax debate as an indication that after the election both the Conservatives and Labour will be "forced to be up-front" about their ultimate tax and spending policies.

Taylor would like to see radical measures to both give tax cuts to the middle class, which he sees as electorally "attractive", at the same time as boosting the government coffers in a "more re-distributive tax system".

"I would have liked to leave open the possibility of a simultaneous raising of the upper band of tax and the tax threshold. If done right, it would mean a middle class tax cut, in the short term, which is attractive because of its electoral power. But it means because of fiscal drag - the process by which as we become more affluent more of us pay higher taxes - you can have a more re-distributive tax system," he said.

In the medium term he proposes a 50/50 settlement - where those earning over £50,000 pay a 50 per cent tax rate.

He says if Labour adopted such a plan it could effectively break a pre-election pledge without angering voters.

"I don't think Labour has any plans to do this, but if a Labour chancellor was to come along in three years time and say, 'you know we promised not to raise taxes but we've got this plan which would mean a cut for everyone but the super-rich'. Then the electorate may well say, 'Fine, formally speaking you may have broken your promise but it's a good idea'."

He says Labour has already done this. "Don't forget in 1997, Labour said it would not be spending much more money and its has spent bags more. Nobody says you have 'you have broken your promise' about spending money because they like it," he said.

In a plea to politicians to abandon the crowd consensus on the issue, Taylor said that voters would respond well to a radical spending debate in the next term.