The wealthy individuals who appear to be enjoying the country's choicest property virtually tax-free, thanks to their exploitation of legal loopholes include a number of Labour party donors, as well as the former Tory prime minister Margaret Thatcher, an influential Saudi prince and Mohamed Al Fayed, the controversial owner of Harrods and Fulham football club.

Whether they possess luxury penthouses in Mayfair or mansions in the Cotswolds, many of their expensive homes are registered as belonging to offshore trusts with concealed beneficiaries.

A land registry review last year called on the lord chancellor to outlaw such behaviour as an abuse, saying "it flies in the face of the principle [that] ownership ... of all properties should be in the public domain". But Lord Irvine, the lord chancellor, who is responsible for the land registry, has so far not acted.

Specialist accountants told the Guardian such offshore registration was a classic method of escaping some or all of three kinds of heavy tax which ordinary homeowners must pay - inheritance tax, stamp duty and capital gains tax.

The computer tycoon David Potter, for example, owns not only his London house but also Rush Manor, a lavish home counties retreat by the Thames.

His fortune, despite recent collapses in the value of internet enterprises, is calculated at �98m.

We estimate that he may be avoiding liability on Rush Manor for his heirs of inheritance tax of around �600,000; liability of �80,000 in stamp duty on a sale; and capital gains tax on the profit he would make if he sold the mansion, originally purchased in 1989, of at least �160,000.

However, Mr Potter was one of the few rich people willing to speak to us on the subject.

When we asked why Rush Manor was not registered as owned by him but by the Ropanom Nominee Corporation, care of a London address which proved to be that of his Psion company lawyers, Paisners, he told us it was "complicated".

He said he personally paid tens of millions of pounds in taxes. But when we asked if his home ownership was registered offshore to avoid tax, he replied: "No comment".

Mr Potter, a Labour favourite and �90,000 donor who gave a 1999 lecture at Downing Street on wealth creation, also uses a second controversial tax loophole by claiming to be "non-domiciled". He has lived in Britain for more than 30 years, but was born in South Africa. As the Guardian recently revealed in a series by Nick Davies on Hans Rausing, Britain's richest man, many wealthy people can live here virtually tax-free by claiming that their true home lies elsewhere.

In the eyes of the Inland Revenue, they therefore have "non-domicile status". Although Mr Potter will pay tax on his UK income from Psion, he does not need to pay tax on income and assets he keeps abroad.

Land registry records show the same pattern in the case of a number of high-profile recent donors to the Labour party.

We found:

� a Panama company owning the north London house of pharmaceuticals tycoon Tony Tabatznik;

� an offshore company listed as owning the�9m summer palace occupied by Indian steel magnate Lakshmi Mittal;

� an offshore trust holding the Grosvenor Square flat of the drug manufacturer Isaac Kaye;

� a Jersey trust company listed as owning the Hampstead home of businessman Uri David.

Another donor, financier and philanthropist, Christopher Ondaatje, has given �2m to the Labour party. For 17 years his second house has been Glenthorne, a coastal mansion in north Devon.

Yet although he has written lyrically about his feeling of "coming home" from Canada by buying it, the 93-acre estate is in fact in the name of the offshore Exmoor Ltd.

All these men claim non-domicile status. None wanted to comment on the allegation that they are avoiding tax liabilities on their UK homes.