The firm signed off accounts in Cyprus, meaning assets such as mansions, private jets and superyachts worth £508m disappeared from the former Chelsea owner’s company accounts.

BBC Panorama claimed the accountancy firm appeared to have also advised on a tax evasion scheme to spare the Russian a huge tax bill through the falsification of documents.

Barrister Andrew Mitchell KC, an expert in financial services law, told the BBC the evidence showed the scheme was “straightforward” tax evasion, adding “it couldn’t be more blatant”.

Lawyers for Abramovich said he denied the allegations that he evaded tax or concealed his fortune, stating Panorama provided no evidence he knew about, authorised or directed the falsification of any document, and adding that it was legitimate to rely on advice from professionals.

Deloitte Cyprus said it “acts in compliance with applicable laws” following a joint investigation by the Bureau of Investigative Journalism and Panorama. The Russian is believed to be living in Turkey following Russia’s invasion of Ukraine in 2022, because it allowed him to evade sanctions.

The Channel Island of Jersey has frozen £5.4bn of his assets, with a criminal probe still underway four years on. Abramovich’s lawyers told Panorama the Jersey investigation was “undertaken for political purposes” and denied the allegations.

The joint investigation raised questions about the integrity of some of his professional advisers. It reported that the evidence was contained in a leak that includes thousands of documents about the oligarch’s finances and the operations of a network of offshore companies administered from Cyprus, a popular tax haven with Russians, between 1995 and 2022.

The BBC outlines the investigation in its new episode, titled “Panorama: Roman Abramovich: How He Hid His Fortune”, including how he made his fortune from a corrupt deal to buy a Russian state-owned oil company in the 1990s, before selling it back to the government a decade later for billions in profit. He previously said the allegations of corruption in the deal were false.

Journalists explain that the profits of the deal were funnelled into a web of offshore companies, administered from Cyprus by a corporate services firm called MeritServus. Until 2005, MeritServus was owned by Deloitte, and following their split the companies worked closely together with associates of Abramovich to manage his fortune.

It claimed the documents were falsified, finding backdated paperwork dating from 2006, after a new law in Cyprus meant company accounts had to be made public, which threatened to expose the secrets of his wealth.

It found that in 2003 Deloitte Cyprus drew up accounts for his Cypriot company Amiscan Investments, but they were not finalised. They detailed $672m of luxury property: a mansion in West Sussex, a London flat, an estate in the south of France, private jets, helicopters and a small fleet of boats the oligarch owned at the time, including the $147.5m Pelorus.

The investigation claimed that associates of Abramovich and MeritServus appeared to have been advised by Deloitte Cyprus on a tax evasion scheme to secretly draw up plans to get around the new transparency law by moving the assets to Amiscan’s parent company in the British Virgin Islands (BVI), where they could stay hidden, with the transfer backdated by three years to make it look like it happened in 2003.

Deloitte Cyprus said it was satisfied that the backdated documents were “adequate” to allow it to remove the assets from Amiscan’s accounts, leaving only $75,000 of assets remaining following the 2006 sign-off.

Panorama reports that an email from MeritServus in August 2005 to associates of Abramovich said the plan was devised following a meeting with Deloitte, adding: “If you agree, it will be necessary to change the financial statements and the accounting records accordingly for 2003 and 2004.”

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.

Andrew Mitchell KC told Panorama 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, Electus Investments, but this time based in the more secretive BVI, where accounts did not have to be made public.

Andrew Mitchell KC told Panorama: “What you take away from it is there are professionals who are prepared to bend and break the rules of their profession and therefore the law. And it looks like they will stop literally at nothing in terms of creating documents.”

Vladimir Milov, Russia’s deputy minister of energy as Abramovich built his empire and now a critic of the Russian government, said: “For each Russian oligarch or Middle Eastern or African corrupt oligarch, there’s an army of Western financial and legal services and PR services providers, who are easily taking money for whitewashing their reputation and making them look normal.”

Denying any wrongdoing, Roman 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.