On Wednesday Alistair Darling for the first time put a figure on the cost to taxpayers of his efforts to rescue the banking system and restart lending, saying it could be as much as £50bn ($72bn), or 3.5 per cent of gross domestic product. In a more hopeful scenario, the Treasury says £20bn could be lost.

But behind these as yet unrealised losses lurks an even bigger mountain of public exposure to the financial sector. More than £1,000bn in public cash has been put at risk since Northern Rock was bailed out in September 2007, the Treasury estimates.