LONDON — The British government slapped a one-time 50 percent tax on fat bank bonuses o 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.