LONDON: More than 100 of Britain’s richest people have been caught hiding billions of pounds in secretive offshore havens, sparking an unprecedented global tax evasion investigation.

George Osborne, the chancellor (finance minister), warned the alleged tax evaders, and a further 200 accountants and advisers accused of helping them cheat the taxman: “The message is simple: if you evade tax, we’re coming after you.”

Her Majesty’s Revenue & Customs (HMRC) warned those involved, who were named in offshore data first offered to the authorities by a whistleblower in 2009, that they will face “criminal prosecution or significant penalties” if they do not voluntarily disclose their tax irregularities, as the UK steps up its efforts to clamp down on avoidance ahead of the G8 summit in June.

The 400-gigabyte cache of data leaked to the authorities is understood to be the same information seen by the Guardian newspaper in its Offshore Secrets series in November 2012 and March this year. It reveals complicated financial structures using companies and trusts stretching from Singapore and the British Virgin Islands to the Cayman Islands and the Cook Islands.

The Treasury is working in collaboration with American and Australian tax authorities in the biggest ever cross-border tax evasion investigation, and warned that the alleged evaders may be publicly named and shamed if they fail to come clean and explain their tax affairs.

Osborne described the data as “another weapon in HMRC’s arsenal” in the fight against global tax evasion. HMRC added it “reveals extensive use of complex offshore structures to conceal assets by wealthy individuals and companies”.

The Revenue said it was continuing to analyse the material, the equivalent of more than 200 lorry-loads of printed A4 sheets, but it has already “identified over 100 people who benefit from these structures”. A number of those “had already been identified and are under investigation for offshore tax evasion”.

It urged those who use offshore tax structures to urgently review their taxation arrangements to ensure they comply with the law, and encouraged those that don’t to ensure “early disclosure of tax irregularities. Failure to do so may result in a criminal prosecution or significant financial penalties and the possibility of their identities being published,” HMRC warned.

It is also investigating more than 200 UK accountants, lawyers and other professional advisers named in the data as advising the wealthy on setting up the elaborate offshore tax arrangements. HMRC declined to name any of the individuals, advisers or companies it is investigating.

An HMRC source said it was first offered a “taster” of the cache in 2009, but received the bulk in late 2010. A spokesman declined to state if it paid a reward to the whistleblower.

The Guardian, BBC Panorama and the International Consortium of Investigative Journalists (ICIJ) have been releasing details of UK citizens and companies acting as offshore middlemen.

Gerard Ryle, director of the ICIJ, said he expected the collaboration between taxmen in the UK, US and Australia to lead to “the largest tax investigation in history”.

He added: “We know from the data we obtained there are names of people from more than 170 countries. Some are prominent citizens — politicians, celebrities, businessmen, the elite of some societies.

“To have three major tax agencies collaborating — with the possibility of many more doing the same — is potentially a major blow to the secrecy of offshore jurisdictions.”

Among those identified by ICIJ data in the joint investigation was James Turner of York-based company formation agents Turner Little, who told undercover reporters how to set up a foundation in Belize: “It doesn’t link back to you, it doesn’t link back to your family. So it gives you complete confidentiality.”

A representative of Atlas Corporate Services, another company run by Britons but operating from Mauritius, explained to reporters how to avoid tax on a hypothetical £6m sitting in a Swiss bank account. He suggested, “off the record”, that they use an offshore entity in Panama. “If there’s a tax issue... they won’t disclose any information on that foundation under Panamanian law,” he said.

Another middleman, Russell Lebe of Readymade Companies Worldwide, advising a reporter posing as an Indian businessman, assured his client that “If we were approached by the Indian tax authority... and they’re doing tax evasion, we wouldn’t give a monkey’s.”

The Guardian, in its investigation, identified 28 individuals with ties to the UK acting as “sham” directors for more than 21,000 companies across the world, keeping the true owners of the companies off official paperwork and thus making them invisible to authorities.

However, there is no suggestion that any of the individuals identified in the Guardian/ICIJ investigation are among those being examined by HMRC.

Jenny Granger, HMRC’s director general for enforcement and compliance, cautioned that not all the individuals using offshore accounts were seeking to evade tax. “There is nothing illegal about an international structure, especially in a globally integrated economy and these arrangements may be perfectly legitimate and may have already been declared to HMRC,” she said. “However, they may involve tax evasion, avoidance or other serious offences by taxpayers. What has got to stop is using offshore structures to illegally hide assets and income.”

David Cameron, the prime minister, has pledged to make tackling the “staggering” levels of tax evasion a key priority of the UK’s presidency of the G8 this year. The EU will hold a summit on tax evasion on May 22. It will be followed by a G8 summit under British chairmanship in June.

By arrangement with the Guardian