The world's biggest accountancy firm helped in falsifying documents to hide Roman Abramovich's wealth and evade tax, BBC Panorama has found.
Accounts signed off by Deloitte in Cyprus meant assets such as superyachts, mansions and private jets worth $672m (£508m) vanished from Abramovich's company accounts.
Deloitte also appears to have advised on a scheme to evade a huge tax bill on the oligarch's wealth through the falsification of documents.
Barrister Andrew Mitchell KC, an expert in financial services law, said Panorama's evidence showed the scheme was "straightforward" tax evasion and "it couldn't be more blatant".
Roman Abramovich's lawyers said he denies that he evaded tax or concealed his fortune. They said Panorama provided no evidence he knew about, authorised or directed the falsification of any document and said it was legitimate to rely on advice from professionals.
Deloitte Cyprus said it "acts in compliance with applicable laws".
The findings are being revealed following a joint investigation between BBC Panorama and the Bureau of Investigative Journalism.
For nearly two decades, Roman Abramovich was the most famous Russian oligarch in the UK, welcomed in London where he bought Chelsea Football Club in 2003 and bankrolled the club's most successful period. But after Russia's full-scale invasion of Ukraine in 2022, he was sanctioned by the UK government as an associate of President Vladimir Putin and forced to sell the club.
Abramovich has been reported to be living in Turkey, a safe haven for him because it did not apply sanctions. In the Channel Island of Jersey, £5.4bn of his assets remain frozen and a criminal investigation has been going on for four years.
The oligarch's lawyers told the BBC he denies the allegations in the Jersey investigation and says it was "undertaken for political purposes".
Our investigation has uncovered new evidence about the way Abramovich's business empire has been run. It also raises serious questions about the integrity of some of his professional advisers.
Lord Sikka, professor of accounting at the University of Sheffield, said it was a "violation of a statutory duty" and "pretty scandalous" to misrepresent transaction dates and not to publish true and fair accounts.
Panorama also found evidence that backdated documents had been used in a tax evasion scheme.
This involved another one of Abramovich's Cypriot companies, Kravin Investments, which had almost $2bn in the bank in 2003.
It had been earning large amounts in interest, so Roman Abramovich's company should have been paying tax on that interest to the Cypriot authorities.
But his advisers had another idea to rewrite history.
An email from MeritServus in August 2005 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 read.
MeritServus informed the oligarch's Russian associates, that Kravin's "Provisional Tax Assessments for 2005" would be filed showing the company earning no income from the money.
A month later, an associate in Russia emailed in response, "I agree".
MeritServus then created a series of backdated documents which falsely purported to show that 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.
Deloitte wrote in a September 2005 email that "certain changes will have to be made to the accounts (bank account balances, interest income, profit and loss account)" because of the backdated changes to Kravin's assets.
So Deloitte was apparently advising on the falsification of financial records. The evidence suggests it allowed Abramovich's company to evade millions in tax.
Andrew Mitchell KC told us the scheme was "straightforward" tax evasion. "If it happened in England and it was in the millions, His Majesty's Revenue and Customs would be thinking about a prosecution," he said.
Cypriot MP Christos Christofides said that if tax had been evaded, then "we have to see how we can get our money back and how, if some people are guilty, they have to pay the price according to our law."
In addition to the documents which Deloitte helped to backdate, Abramovich's advisers at MeritServus falsified a third set of records. This involved the oligarch's huge business fortune - more than $10bn - held by a company called Electus Investments in Cyprus.
MeritServus backdated documents to make it look like Electus did not own the fortune.
It used another Abramovich company with exactly the same name of Electus Investments, but this time based in the more secretive BVI, where accounts did not have to be made public.
Emails show that in 2006, MeritServus acknowledged that the fortune was owned by Electus Cyprus. But again it proposed to rewrite the past, this time "to transfer the beneficial ownership to Electus Investments in the BVI" and make it look as though the change had happened three years earlier.
MeritServus signed and backdated documents which falsely purported to show the BVI company had owned the $10bn fortune since 20 November 2003. So when the accounts of the Cypriot company with the same name were made public, it appeared to have assets worth just $145,000.
Roman Abramovich's billions had been made to disappear.