The British government slapped a one-time tax of 50 percent on fat bank bonuses on Wednesday as it tried to win over recession-weary voters ahead of a looming general election.

But Treasury chief Alistair Darling's plan to exact payback for the crisis that led Britain into its worst recession since World War II faced opposition criticism that it was at best political spin and would do little to raise revenue _ and, at worst, potentially damage London's standing as a financial center.

Darling's overall pre-budget report, in which he acknowledged that the economy will shrink more this year than previously predicted and increased government borrowing forecasts, was also criticised as likely to do little to aid Britain's sluggish economic recovery.

With Prime Minister Gordon Brown's government trailing the opposition Conservative Party in opinion polls ahead of an election that must be held by June, the tax proposal was clearly aimed at voters who funded a bailout of the banking system at the height of the crisis last year.

"There are some banks who still believe their priority is to pay substantial bonuses to some already high-paid staff," Darling told lawmakers Wednesday as he delivered the government's pre-budget report in Parliament.

"Their priority should be to rebuild their financial strength and to increase their lending," he added. "If they insist on paying substantial rewards, I am determined to claw money back for the taxpayer."

But opposition politicians, the banking industry and many economists said the tax, to be levied on 2009 discretionary bonuses of more than 25,000 pounds ($40,800), was political spin that would do little to raise revenue and could turn investors away from London.

Monument Securities strategist Mark Ostwald said it was a "totally gratuitious" measure that would raise around 550 million pounds ($890 million) _ a drop in the bucket of the increased forecast of 178 billion pound government borrowing this year and 176 billion pounds next year.

The tax will be imposed on the pool of bonuses paid by a bank, rather than individual payments and it will be paid by the bank _ not the recipient of the bonus.

"Viewed from abroad, London may well look now like a significantly less attractive place to build a business," said British Bankers' Association Chief Executive Angela Knight.

Rod Roman, Financial Services Partner at Ernst & Young, said that the one-off tax could damage future expectations.

"Those investing in the U.K. will have to double guess the actions of future governments and in this situation, low tax rates will no longer attract companies to the U.K.," Roman said.