ALISTAIR DARLING is under pressure to announce a bank tax in his budget this week after the Conservatives said they would punish the City with a new levy.

Yesterday David Cameron pledged to introduce a tax on the banks unilaterally if no international agreement could be reached. It was a u-turn by the Tories, who had previously insisted that for Britain to act alone would push the financial services industry offshore.

Tory officials insisted that they expected international agreement on a bank levy, but that in its absence there was a case for a national tax to compensate taxpayers for their rescues of the banks and provide insurance against future crises. They cited the case of Sweden, which introduced a “stability fee” on its banks last year.

Treasury officials insisted yesterday that in the budget the chancellor would reaffirm his commitment to pushing for an internationally agreed levy.

Doug McWilliams, chief executive of the Centre for Economics and Business Research, said: “It would make sense from the government’s perspective for the centrepiece of the budget to be a £5 billion tax on banks to fund a series of pre-election giveaways — £5 billion is roughly the amount banks save from not being subject to Vat and other indirect taxes.”

However, Lord Myners, the City minister, said: “This ill thought-out Tory briefing has all the hallmarks of a plan made up on the hoof. This kind of tax on bankers needs to be international.”

Tory officials refused to be drawn on the precise size of any unilateral bank tax, though they said yesterday it would be “a lot less” than £5 billion.

The British Bankers’ Association attacked the proposal. “We are ready and willing to work for change,” it said, “but we believe any further reforms need to be timely, considered and internationally co-ordinated so they do not restrict credit to individuals and businesses as the recovery picks up speed.”

Darling is set to use most of the undershoot of several billion pounds in this year’s public borrowing to lower the profile of the budget deficit for future years. The expectation among analysts is of a £10 billion undershoot on the £178 billion borrowing forecast.

He will warn, however, that markets should not expect a sudden decline in the deficit, despite the government’s commitment to halving it over four years. The Debt Management Office, which manages funding on behalf of the Treasury, will issue about £190 billion of gilts in the 2010-11 fiscal year.

Darling will also unveil a growth strategy based on “industrial activism” and support for jobs. It will include growth plans for every region, a drive for closer co-operation between universities and business, and ambitious plans for infrastructure spending.