Thousands of British people who have moved or retired abroad could be chased for tax going back six years following a court ruling last week.

The landmark decision changed the way Revenue & Customs calculates whether you are deemed to be resident in Britain, and therefore whether you must pay UK tax on your worldwide income and gains.

In effect, it means that people who live abroad but visit Britain regularly will be able to spend fewer days in this country before they fall foul of the taxman.

Mike Warburton of Grant Thornton, an accountant, said: “This ruling could affect ordinary middle-class families who have moved or retired abroad but who come back to Britain regularly for domestic reasons, such as visiting their grandchildren. The residence rules concern them just as much as millionaires who jet into Britain for business but spend the rest of their time in tax havens.

“They will have to be much more careful when they calculate how many days they spend in the UK, and the onus is on them to prove to the taxman that they are not resident. My advice would be to keep your plane tickets and a diary from now on.”

The Revenue’s rules state that even if you settle abroad, you could still be deemed a British resident and subject to tax on your worldwide income and gains if you spend time in Britain.

You must have spent an average of no more than 90 days a year in Britain over four years to be non-resident.

Until now, the taxman has excluded the days you arrive and leave, but in last week’s ruling against Robert Gaines-Cooper, a businessman who said he was a Seychelles resident, the court decided they should be taken into account. This significantly increased the amount of time he spent in Britain — from 79 to 128 days in one tax year alone.

Accountants said the ruling was part of a wider clampdown on people who retire or work abroad. The chancellor is seeking £3.5 billion from taxpayers to plug a hole in the public accounts, and the Revenue has identified overseas assets as an easy target.

It is probing the estates of people who owned property abroad, for example, to make sure their families have paid sufficient inheritance tax.

And it is looking more closely at all businessmen, not just millionaires, who quite legitimately work abroad most of the time, but who regularly return to Britain for meetings and to see relatives.

Martin Rimmer of the Fry Group, another firm of accountants, said: “The upshot is that Revenue & Customs seems to be adopting a much stricter attitude towards endorsing expatriate status and the guidance provided by its own publications can no longer be relied upon. Saving UK tax by moving abroad is becoming much more difficult, and the taxman is likely to continue to pay greater attention to people wanting to adopt non-residence status.”

About 1m people have retired abroad, according to Lombard Street Research, and this could rise to 1.3m by 2025. Another 280,000 households own a second property abroad, and many intend to settle there when they retire. One in five of us — nearly 10m adults — is considering fleeing Britain’s rising taxes and cost of living.

Justin Woods, 41, and his wife Andrea, 32, are among them. Justin used to be an IT consultant in the City, but gave it up to run a building company when he had children. He has become increasingly disillusioned with Britain, and is moving to Moulin, central France, next year with his children — twins Stephanie and James, aged three, and Harvey, one. “Our council tax is £2,000 and our house insurance is the same, compared with just £250 and £100 respectively in France,” he said.

The family will have to think carefully about the residence rules, though, because they will retain a small business in Britain and will be visiting a sick relative regularly.

Accountants have scores of ordinary clients who could be affected by the ruling. Warburton said: “I have many clients who have retired abroad and might spend most of the year there, but who come back regularly to see their families. The initial reason for moving may have been the climate or their health, rather than tax, but because they have significant assets in offshore accounts, perhaps from the sale of a British property, they are careful to spend fewer than 90 days a year here so they are not subject to UK tax, although they could be subject to local taxes in the foreign country.”

They could now find they are still considered resident and owe more money to the Revenue. Even where people are well below the 90-day limit, the Revenue could still chase them to recalculate their status on the new basis.

For more on tax visit www.timesonline.co.uk/tax