Wealthy ex-pats who have previously avoided paying UK income tax by limiting the time they spend in the UK could now find themselves under increased scrutiny from HM Revenue & Customs.

According to accountants Deloitte, tax exiles who claim to be non-resident but have continued to visit the UK for up to 90 days per annum on average may not be able to claim exemption from UK tax, and should seriously reconsider their position.

In addition, companies sending employees to work overseas should reconsider the employee's likely residency status where he or she continues to spend substantial amounts of time in the UK.

Under the current HMRC guidance, in order to achieve non-residency status, and therefore qualify for the associated tax breaks, individuals must "leave" the UK and then not spend more than 90 days on average here per UK tax year while they are abroad.

However, a recent decision by the Tax Tribunal (Special Commissioners) could reduce opportunities for those looking to exploit the loopholes in the tax system.

In the Tribunal case, HMRC argued successfully that British-born businessman and multi-millionaire Robert Gaines-Cooper could not be classed as non-UK resident, despite spending less than an average of 90 days in the UK per UK tax year, because he maintained sufficiently strong links with the UK. At the Tribunal he had no defence against HMRC's decision that he was not entitled to take advantage of the 90 day average rule, because the rule is a guideline only and is not the law.

Mr Gaines-Cooper had substantial business interests in several countries and spent a lot of time travelling. But he considered himself to be resident and domiciled in the Seychelles since 1976.

However, because of his UK origin, citizenship, pattern of regular visits and long standing UK connections, including a UK home where his Seychellois wife lived for nine months of the year and which he visited regularly, the tribunal decided that he was ordinarily resident in the UK and only "occasionally" resident abroad. They deemed that he had never truly "left" the UK and was therefore still chargeable to UK tax on his worldwide income.

Campbell Oswald, partner and head of Deloitte's international assignment services practice in the Midlands, said: "Most expatriate employees who live and work outside the UK in full time employment for at least a complete UK tax year will remain non-UK resident. Employers should however check at the outset of an international assignment so the risk of continued UK residency and the associated implications can be minimised.

"The Gaines-Cooper case will nonetheless raise difficulties for individuals who were acknowledged to be non-resident in the UK in the past, possibly many years previously, but who have retained significant UK connections since moving abroad.

"Those at particular risk are commuters and UK tax exiles for whom regular visits to the country are part of their lifestyle – the so called 'Monaco Millionaires'.

"The ruling by the Special Commissioners will encourage HMRC to examine their overall lifestyle, including any relevant links with the UK, whether social, family, business, cultural or sporting."

"In the future, to establish non-residence, tax exiles may need to break further ties with the UK, including disposing of any UK home and reducing visits to an absolute minimum – possibly not visiting the country at all for at least a complete tax year."