The HQ of Kleinwort Benson Bank Limited, 14 St George Street, London.

In another major blow to international personal tax evasion, the names of over 20,000 wealthy offshore clients of a private bank in the Jersey island tax haven, have been leaked to the International Consortium of Investigative Journalists.

The individuals include donors to the British Conservative Party, which has proposed restrictions on tax havens, and some of the most prominent people in British life.

The ICIJ has exclusively allowed The Guardian newspaper to analyse the names of the clients of the Jersey, Channel Islands branch of Kleinwort Benson, the London firm which provides private banking and wealth management services to international private clients, families, business owners.

In the interests of transparency, ICIJ said The Guardian will publish some of their findings over the coming days, detailing the offshore links of political donors; international celebrities; judges; sportsmen; businessmen; and British aristocrats.

Names range James Dyson, Britain's most famous inventor, to Hollywood actor Mel Gibson. Today The Guardian identifies party donors who over the years have paid more than £8m to the Conservative Party.

ICIJ says one of the recipients of donations is Andrea Leadsom, Britain’s newly-promoted financial services minister, who has run into a “Cash for Office” allegation after she told The Guardian she was unaware of the size of large offshore donations to the Conservatives made by her own  family.

The ICIJ has previously published secret internal records of offshore companies in the British Virgin Islands.

Gerard Ryle, ICIJ director, said on Thursday: “We make this information available not because what we found is illegal but because we think most people would think it unfair. Tax havens allow some people to play by different rules.”

The group says that George Osborne, UK chancellor, announced in April: “If you’re evading tax offshore, there is no safe haven and we will find you.”

But The Guardian findings contradict this picture of illegality. Many Jersey loopholes used by wealthy Britons to pass on their fortunes appear from The Guardian’s research to have been perfectly legal.

Both the ruling Conservatives and to a lesser extent the Labour opposition party have benefited by political donations from such individuals.

There is no published register of trusts or offshore holdings, and the rich appear to have often been able discreetly to avoid taxes, particularly inheritance tax, in ways too expensive for smaller people to use.

Kleinwort Benson Investors is an established institutional asset manager with headquarters in Dublin.

Wikipedia says that the earliest known Kleinwort to go into banking was 24-year-old Hinrich Kleinwort who, in 1786, set up a partnership with Otto Mueller in Holstein to finance trade with England. In the very same year, Robert Benson joined with William Rathbone IV of the existing house of William Rathbone & Co. to form Rathbone & Benson, a Liverpool business trading mainly with America. Over the course of the 19th century, the Benson family grew its wealth through railway finance in Britain, Europe and America. The Kleinworts established a successful trading business in Cuba, profiting from the expansion of the H. Upmann and Sons cigar business.

The Kleinworts and the Bensons merged their firms in 1961 to form Kleinwort Benson. Kleinwort Benson then became one of the first merchant banks to establish itself in the Channel Islands, having set up offices in Jersey in 1962 and Guernsey in 1963.