Conservative leader William Hague added to the speculation that the party's flagship tax policy was due for a shift when he described its "guarantee" to reduce the burden of taxation as an "aspiration".

During a press conference on the party's drug policy on Tuesday Mr Hague was asked whether the Tories were sticking to their promise to cut the tax burden over the course of a Parliament whatever the economic circumstances.

He replied: "The tax guarantee is about reducing taxation and Conservatives believe in reducing taxation.

"We've got a government that is increasing taxation, by taxing people by stealth on their petrol, on their pensions, and their marriage and their mortgage.

"We offer them a powerful argument that a country that wants to compete in the future has got to bring down the burden of taxation over time - and that is the aspiration expressed in the tax guarantee."

Mr Hague insisted that the "tax guarantee", unveiled only last autumn, was still a guarantee but said the issue needed to be discussed further with the party's new shadow chancellor, Michael Portillo.

First hint from Portillo

The Tory leader's signal that the policy was due for a shift followed a weekend radio interview by Mr Portillo in which he appeared to downgrade the status of the pledge, which was drawn up under his predecessor Francis Maude.

Asked whether he supported the guarantee to cut taxes that as a share of national income under a future Conservative government, Mr Portillo said: "Well, I am happy to discuss that."

His comments sparked speculation that Mr Portillo may wish to drop or alter the pledge, following his reversal last week of Tory hostility to the minimum wage and Bank of England independence.

On Tuesday Mr Hague added: "Over the course over the coming months and in the run up to the general election, we will give more details of what that means, and we will be more specific."

Challenged as to whether the guarantee was now an aspiration rather than a promise, the Tory leader replied: "The guarantee to reduce taxation is a guarantee, but of course there are further details to be added to that, and there is a lot to discuss with the new shadow chancellor and the whole team."

He declined to say whether his phrase "over time" related to the life of a Parliament or the economic cycle.

'Tory turmoil'

Cabinet Office minister Ian McCartney accused Mr Hague of "weak leadership of the worst kind".

"The Tories are worse than ever," he said. "They lied about taxes in government 22 times and now they have been caught out lying about taxes in opposition."

"What's also clear is there is turmoil at the heart of the Tory Party."

Liberal Democrat Treasury spokesman Matthew Taylor claimed the Tory tax pledge was "a fantasy policy" which was never a serious proposal for government.

"The 'tax cut guarantee' was the linchpin of a Tory manifesto which is now starting to unravel," he said.

"The suspicion will be that Michael Portillo is chucking out William Hague's policies in preparation for chucking out William Hague."