It was revealed by The Sunday Times last month that Jennifer Moses, the American former investment banker who now advises Gordon Brown on special projects, arranged an offshore mortgage on her north London home through Barclays Private Clients International, based on the Isle of Man.

Thousands of other wealthy business people who are resident but not domiciled in the UK also use offshore loans. They allow you to pay the mortgage interest out of offshore income rather than bringing the money onshore, where it would be subject to 40% income tax.

It had been thought such arrangements would be permitted to continue in spite of the government’s crackdown on perks available to non-doms.

Bowing to pressure from critics who said the move would drive talented and wealthy individuals offshore, chancellor Alistair Darling watered down the proposals so that those who took out loans before April could continue their arrangements — a legislative move known as “grandfathering”.

Last week, however, accountants raised concerns that loans taken out after 2005 would still be caught by the rules, because many of those mortgages were not secured against the property.

Instead, banks have tended to secure loans against a guarantee which was in turn issued against the property to comply with Financial Services Authority rules.

Carolyn Steppler of accountants KPMG said: “We would like to see a relaxation of the rules as the grandfathering provisions are very tightly drawn. It is clear most offshore mortgages taken out after 2005 will not fall within the grandfathering.”

She added: “Clients are particularly concerned about this. We have argued with HM Revenue & Customs on their behalf that they already arranged their financial affairs to take advantage of the legislative rules.

“Furthermore, they may not be in a position to remortgage, and this is a particularly unfavourable option in the current lending climate.”

The proposals are to be debated in parliament this week.

Under the changes announced by the chancellor in April, non-doms must pay a £30,000 annual levy to remain outside the jurisdiction of HMRC, or face close scrutiny of their affairs.

Critics of the measures, laid out in a consultation paper, asserted that the proposals will force businesses run by non-doms to move overseas and discourage leading international businessmen from working in Britain, which forced the government’s climb down.

Steppler added: “It does appear that HMRC are listening and they might introduce some changes.”