Alistair Darling may have climbed down on his original reforms to the taxation of non-domiciled taxpayers, but wealthy individuals and big business have already mounted a second assault on the changes.

Last week HM Revenue & Customs acting chairman Dave Hartnett wrote a letter 'clarifying' the taxman's position towards non-doms. The letter provided assurances that HMRC would not wade into the overseas tax arrangements of non-doms and that gains already accrued in offshore trusts would not be taxed when paid into the UK.

Hartnett confirmed that money brought into the UK for the £30,000 levy non-doms will have to pay to keep foreign income tax free will not be taxed. Art work brought in for museums and galleries would not be taxed either.

The concessions by the government did address the most serious concerns of the City, which emerged when draft legislation was first released in January, but business remains unsatisfied.

'Wealthy resident non-domiciliaries will continue to review their affairs and will still keep their options open, potentially resulting in a decision to depart the UK, and they and their advisers still face a mammoth task to reorganise their affairs by that date if they decide to stay,' Ernst & Young tax partner Andrew Tailby-Faulkes said.

Darling and the Treasury have already had to fend off attempts to delay the final implementation of the regime, set to kick in from 5 April.

But government is almost certain to continue facing fierce lobbying to make further concessions, with the City expected to pay particular attention to the new rules for taxing gains realised by non-doms in off-shore trusts, which will now be caught in HMRC's net.