Chancellor Alistair Darling faces a humiliating climbdown after it was revealed figures he used to attack Tory proposals on non-domiciles should be treated with ‘a great deal of caution’.

In a letter to the Shadow Chancellor, George Osborne, the permanent secretary to the Treasury, Nicholas Macpherson, acknowledged that ‘there is no complete data set for the unremitted foreign income of non-domiciles’, reports the Daily Telegraph.

The row erupted after Darling this week dismissed Osborne’s costing of a £25,000 non-dom tax proposal, saying: ‘The Treasury’s initial estimate, on best information available, is that just 15,000 current non-domicile residents have foreign income in excess of £62,000.’

But the Treasury document that set out the costings said: ‘All figures are best estimates but need to be treated with a great deal of caution due to the lack of available data or evidence.’

The Tories plan to use the tax to pay for an increase in the inheritance tax threshold to £1m.