The British banking industry rounded angrily yesterday on reports that the government plans to introduce a windfall tax on bonuses, describing the proposals as a punitive measure that would trigger an exodus of financial services companies from London.

Alistair Darling, chancellor of the exchequer, is expected to include a form of “supertax” on bankers' bonuses in his pre-Budget report tomorrow, as the Labour government attempts to set out clear dividing lines with the opposition Conservatives ahead of a general election next year.

While details of any such scheme remain thin, Mr Darling is said to be keen to send a warning signal to banks that are intending to distribute hefty awards to staff in the aftermath of the global financial crisis, when they were propped up by billions in taxpayers' funds.