The Treasury’s analysis of the wealth of non-domiciliaries was thrust to the centre of political debate this week, as the two major political parties traded blows over a Tory plan to tax them more heavily.

Shadow chancellor George Osborne announced on Monday a plan to charge the non-doms a flat £25,000 levy for the privilege of acquiring the preferential tax status, allowing them to avoid tax on offshore income.

Osborne used estimates that as many as 150,000 non-doms might pay the new levy ­ Labour claimed that Treasury analysis showed it would only be as many as 15,000, since the rest would simply pay tax on their overseas earnings.

But in the past the Treasury has insisted that it doesn’t know how much foreign income non-doms have, inviting criticism that it had either made up its 15,000 figure or been withholding information.

Dawn Primarolo, the former paymaster general, said earlier this year: ‘Information is not held on overseas income and gains that do not give rise to a liability in the UK.’

Mike Warburton said the Treasury figure was ‘patently tripe. Nobody knows the actual figures’. He added that the Tory estimates were more likely to be accurate.

The Tories themselves criticised the earlier non-disclosure of figures on non-doms this week. MP Justine Greening claimed the Treasury withheld figures in the past as it was not ‘expedient’, information that it was only too happy to provide now for ministers.

‘What is unacceptable is for government to withhold data from opposition parties for no other reason than that it was not expedient at the time,’ she said.