Accountancy giant Deloitte helped Roman Abramovich avoid tax by hiding cash and assets including luxury yachts, mansions and private jets, leaked documents suggest.
An investigation by BBC Panorama and the Bureau of Investigative Journalism found financial records were allegedly falsified and backdated to shield the Russian billionaire and former Chelsea FC owner from paying millions in tax.
Abramovich’s lawyers deny wrongdoing and say he relied on professional advice.
The leaked documents suggest that advisors at corporate services firm MeritServus shifted Abramovich's £7.5 billion ($10bn) business fortune from a Cyprus company called Electus Investments to a British Virgin Islands company with the same name.
The documents were allegedly backdated to make it appear the transfer had happened in 2003, before new transparency rules took effect in Cyprus in 2006.
When the Cypriot company's accounts were later published, it appeared to hold assets of only £110,000 ($145,000), it is claimed.
Until 2005, MeritServus was owned by Deloitte, which is now the biggest of the 'big four' accounting industry firms. After they split, the two companies worked together with Abramovich's associates to manage his global fortune.
Labour peer and accountancy professor Lord Sikka accused Abramovich's financial advisors of running a 'fiddle factory'.
Financial advisors from Deloitte allegedly helped former Chelsea owner Roman Abramovich to hide billions in cash and assets to avoid paying tax
'The amounts are very significant, very material, and they are being backdated - retrospectively being moved. It is trying to cover up something,' he said.
'What we appear to have here is a fiddle factory. In a fiddle factory, you fiddle the transactions, you backdate them, and that is what appears to have been happening here regularly.
'Companies are required to publish true and fair accounts, and if the transaction dates are being misrepresented, that is not being truthful. That is a violation of a statutory duty. So I think it is pretty scandalous, really.'
Separate leaked documents showed how £507 million ($672m) of luxury property owned by Abramovich through another Cyprus-based company called Amiscan Investments was allegedly 'hidden'.
The property included a mansion in West Sussex, a luxury flat in London and an estate in the south of France, along with supercars, helicopters, private jets and a small fleet of boats including the £111.5 million ($147.5m) yacht Pelorus.
It is alleged that Deloitte removed the property from Amiscan Investments' published accounts through a purported transfer to a company in the British Virgin Islands, then backdated it by three years.
Panorama also claims that Abramovich's advisors helped him to evade tax with another Cyprus-based company, Kravin Investments.
The leaked documents allegedly show they changed the firm's earlier records to make it appear that the company did not own nearly £1.5 billion ($2bn), and therefore had no taxable interest income.
A 2005 email from MeritServus to Abramovich's Russian associates said a plan had been devised after a meeting with Deloitte.
'If you agree, it will be necessary to change the financial statements and the accounting records accordingly for 2003 and 2004,' the email said.
A month later, an associate in Russia replied: 'I agree.'
It's claimed MeritServus then created a series of backdated documents which falsely showed Kravin was only holding and investing the funds on behalf of its parent company in the BVI.
This made it look like Kravin did not actually own the money, so it would not be liable for tax in Cyprus.
Banking and financial services lawyer Andrew Mitchell KC said: 'It's a straightforward and blatant move to protect from legitimate taxation assets which should otherwise have been subject to tax.
'If it happened in England and it was in the millions, His Majesty's Revenue and Customs would be thinking about a prosecution.'
Abramovich was sanctioned after Russia attacked Ukraine. It was claimed he had a close personal relationship with Russian president Vladimir Putin - something Abramovich denied.
He is understood to be living in Turkey, which has become a safe haven for Russian oligarchs because it did not apply sanctions.
Responding to Panorama's allegations, Abramovich's lawyers said he was 'not involved in the day-to-day operation of those companies' and that where arrangements were proposed by professional advisers, 'reliance on that advice was entirely proper and legitimate'.
Deloitte Cyprus said it could not comment on the specific allegations because the documents were more than 20 years old and no longer accessible.