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The 'Panama Papers' appear to have revealed Britain's key role as a link to thousands of firms based in tax havens.

UK firms feature as 'intermediaries' more often than almost any other nation in 11million secret files obtained from Panama law firm Mossack Fonseca.

HM Revenue and Customs is set to probe the documents handed to a global group of journalists, who said they name dozens of world leaders and celebrities using tax havens to hide their wealth.

Much tax avoidance is legal, most of those named have spoken out to deny wrongdoing and Mossack Fonseca says it operates "beyond reproach".

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But Britain's huge role in financial secrecy is piling pressure on David Cameron - who has attacked offshore firms and is hosting an anti-corruption summit in London this May.

Last summer he said prime London property worth billions of pounds was being bought "through anonymous shell companies, some of them with plundered or laundered cash."

He added: "I’m determined that the UK must not become a safe haven for corrupt money from around the world.

"We need to stop corrupt officials or organised criminals using anonymous shell companies to invest their ill-gotten gains in London property, without being tracked down."

Yet despite his pledges the International Consortium of Investigative Journalists (ICIJ), a network of more than 100 news outlets, claims Britain is one of the main go-between nations named in the Mossack Fonseca files.

Vast amounts of data analysed by the ICIJ shows Britain is the second most prominent country where the law firm's middle men operate, with 1,924 identified firms.

It was second only to Hong Kong, which had 2,212.

When it came to the number of "active intermediaries" Britain came third with 32,682, second only to Switzerland with 34,301 and Hong Kong with 37,675.

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And a British overseas territory, the British Virgin Islands, was by far the most popular tax haven state used by firms in the documents.

According to the ICIJ, more than half of all the firms listed - 113,648 - were incorporated in the British Virgin Islands.

That was far more than the 48,360 listed in Panama despite the South American country hosting Mossack Fonseca's headquarters.

The files also name Mr Cameron's late father Ian as allegedly using the law firm to shield his investment fund, ­Blairmore Holdings Inc, from UK taxes.

Papers say his fund was “managed and conducted so it does not become resident in the United Kingdom for UK taxation purposes”.

Opposition politicians have blasted the Tory Prime Minister for failing to act.

Shadow Chancellor John McDonnell said: “The Panama papers revelations are extremely serious.

(Image: REUTERS/Toby Melville)

"Cameron promised and has failed to end tax secrecy and crack down on ‘morally unacceptable’ offshore schemes, real action is now needed.”

Lib Dem leader Tim Farron said: "Despite calling last year for tax havens to open their books David Cameron has since done nothing to ensure that UK Overseas dependencies such as the Cayman Islands, Bahamas and the British Virgin Islands give the transparency that taxpayers deserve.

"Once again the Conservative Government has failed to act until forced to by a scandal.

“Rather than waiting until more headlines force action, the Government should now commission a full independent investigation into the way the Virgin Islands and others have acted, what the UK Government knew and why they have not used their legislative powers to impose the transparency rules they previously claimed to support.

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"For Britain to be associated with this kind of tax avoidance utterly against our values. They are sunny places where shady deals happen."

Robert Barrington, Executive Director of campaign group Transparency International UK, added: "The Panama Papers seem to confirm the evidence from elsewhere that the world's corrupt elite are gaming the international financial system to launder and protect their stolen wealth.

"The only way to stop this grand corruption is through governments, businesses and others coming together and rejecting dirty cash as illegitimate.

"The time has come to stop turning a blind eye to anonymous purchases of luxury property and goods, refuse to issue un-vetted investment visas and create a legal framework that is fit for purpose in detecting flows of dirty cash.

(Image: Getty) (Image: https://twitter.com/mossfon)

“The Prime Minister's Anti-Corruption Summit in May is the perfect opportunity to address these issues.

"But it will only do so if the agenda is ambitious and those who are complicit in grand corruption are not allowed into the room to torpedo effective action."

Mossack Fonseca rejected any accusations of wrongdoing and said it had operated “beyond reproach” for 40 years.