LONDON (AP) — The British government slapped a one-time tax of 50 percent on fat bank bonuses yesterday 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 centre.

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 yesterday 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 ($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 ($890 million) — a drop in the bucket of the increased forecast of £178 billion government borrowing this year and £176 billion 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 UK will have to double guess the actions of future governments and in this situation, low tax rates will no longer attract companies to the UK," Roman said.

Paul Kenny, general secretary of the leading GMB union, however, said that taxing bonuses was "long overdue".

"There is a cultural issue here that needs to be addressed and the public will support the government in dealing with it," Kenny said.

Conflicting economic and political pressures made Darling's task of laying out the government's spending, taxation and borrowing plans a highwire act. As expected, with one eye on voters, he steered clear of major tax increases or spending cuts that could address the hole in the government's finances.

"We were promised a pre-budget report, and what we got was a pre-election report," said opposition Treasury spokesman George Osborne. The Conservatives, who have called for quicker action to bring down the deficit, have pledged to rewrite the budget within 50 days if they win the election.

Britain is facing a heavy hangover from the global credit squeeze because of its huge financial sector, and higher levels of personal debt among consumers. It remains the only major economy still officially in recession.

Darling yesterday acknowledged that the economy will shrink by 4.75 percent this year, significantly worse that the 3.25-2.75 percent contraction he predicted at the time of the full annual budget in April.

He reiterated his belief that the economy will restart growth by the end of the year.

Darling confirmed that retail sales tax would rise back to 17.5 percent, after a year-long cut to 15 percent to aid economic growth, on January 1.