HM Revenue & Custom's ability to cope with advising up to five million migrant workers on how the non-dom tax crackdown might affect them has been challenged by shadow Tory treasury minister Mark Hoban.

The Accountant MP questioned how those on lower incomes will be able to chose which way to be taxed as non-dom as the Commons finished its detailed debates on the Finance Bill.

The choice involves a technical decision whether to be taxed on a ‘remittance basis’, and sacrifice personal allowances, or be taxed on an ‘arising basis’.

Hoban warned: ‘Those calculations will be made not just by high net worth individuals who have access to expensive accountants and lawyers.

‘They will have to be made by migrant workers, seasonal workers, who come to the UK to pick fruit and help in the agricultural industry, people from overseas working in the NHS, skilled migrants entering through Home Office programmes, international students and even Commonwealth citizens serving in the armed forces.’

Treasury financial secretary Jane Kennedy insisted a decision to double the limit on making a tax return on overseas from £1,000 to £2,000 would reduce the problem, and said HMRC would have plenty of time to put new resources in place before the April 2009 deadline.

She promised there would be additional targeted help and guidance,but made it clear she believed ‘the vast majority of low-income migrant workers are either under the [new] £2,000 limit or better off on an arising basis’.

Hoban said later: ‘The original legislation on this was ill thought out and has caused a great deal of uncertainty, even among advisers.

‘I am still not convinced the government knows how unrepresented taxpayers will be able to comply.’

During extended debates Kennedy revealed the Treasury has slashed 400 from the estimate of 3,000 non doms expected to leave the UK as a result of the crackdown, insisting: ‘The UK remains an attractive place to live, work and do business.’

She declined to reveal the estimate methodology resulting in the reduction.