The cost of new tax rules for non-domiciled individuals doesn't add up, says accountancy firm Grant Thornton.
It maintains the Government's consultation on residence and domicile has ignored what the cost of the changes to the UK economy will be and specifically whether the proposals will result in an actual net loss to the Exchequer.
Chris Mills, director, said: "The consultation document is silent on the adverse impact of the cost of the new proposals."
It states that non-domiciled individuals contribute £12 billion to UK GDP and £4 billion in income tax revenues. Some 20,000 individuals are estimated to be affected by the £30,000 levy due to be introduced, which is expected to raise £350 million in 2009/10, reducing to £200 million in 2010/11.
The consultation acknowledges that 3,000 people will become non-resident, which equates to around 15 per cent of the people affected by the levy.
But Grant Thornton believes such a reduction in the numbers of those expected to remain resident in the UK could lead to a significant fall-off in tax revenues.
It states: "If the £150 million reduction in yield from the levy between 2009/10 and 2010/11 is based on 3,000 people becoming non-resident, this indicates that not only would the £30,000 levy be foregone, but would also include an additional £20,000 per annum in other tax revenues.
"This is low compared to the estimated £4 billion in income tax paid by non-domiciled individuals. What if the number leaving or their current tax contribution were higher?
Mr Mills added: "There is a strong margin for error in the Government estimates, which have already been revised downwards since the Pre-Budget Report announcement."
Grant Thornton says the levy may be picked up by employers but this would represent another cost of doing business in the UK.
The super-rich non-domiciled individuals may decide that the levy is not material to their affairs, pay it and continue to remain in the UK. However, these individuals are internationally mobile and it could cause friction in the medium term.
Others will need to weigh up whether it is cheaper to pay the levy or the tax on all worldwide income and gains.
The firm cautions: "As a large number of people work as 'Eurocommuters', they may be faced with being resident and taxed in two countries.
"Whilst there are treaties in place to determine which country takes the tax, it will undoubtedly make things more complicated for these people. The obvious example are transport workers who make a daily or two day trip to the UK on a regular basis. Lorry drivers, Eurostar drivers and ferry staff who are not resident at the moment may be about to get a shock, a tax bill and a lot of hassle."