Leaked documents reveal a money trail linking oligarch Roman Abramovich to two men dubbed "wallets" of President Vladimir Putin.
The former Chelsea Football Club owner has been sanctioned by the UK and EU but has previously denied any financial relationship with the Russian leader.
Now, leaked documents from Cyprus reveal new evidence linking him to a secret $40m (£26m) deal in 2010.
Mr Abramovich has not responded to requests for comment from the BBC.
The secret deal transferred shares in a highly profitable Russian advertising company, Video International - for less than they appeared to be worth - from companies ultimately owned by a trust connected with Mr Abramovich, to two members of Putin's inner circle. They in turn received millions of dollars in dividends.
BBC Newsnight, BBC Verify and Panorama partnered with the Bureau of Investigative Journalism to uncover the revelations as part of Cyprus Confidential - a global investigation led by reporters at the International Consortium of Investigative Journalists and Paper Trail Media.
Confidential records reveal that one of the men involved in the secret deal was Sergei Roldugin, a close friend of the Russian president.
A cellist, Mr Roldugin is the artistic director of the St Petersburg Music House. He has known Vladimir Putin since they were young men in St Petersburg, and is reported to have introduced him to Lyudmila Shkrebneva, whom the future president married in 1983 (they are now divorced). Mr Roldugin is the godfather of their first daughter, Maria.
The second man is another close associate of President Putin - Alexander Plekhov, a biochemist-turned-businessman, also from St Petersburg.
Mr Roldugin and Mr Plekhov have both been accused of being "wallets" for President Putin - secretly holding money and assets on his behalf.
Earlier this year, Swiss prosecutors alleged they were "straw men", and not the real owners of assets in bank accounts set up in connection with the Video International deal.
The court did not identify anyone as the true ultimate beneficial owner of the accounts.
President Putin's stated salary in 2021 was just over $100,000 (£72,700). However, there are rumours his fortune could be worth anywhere between $125bn (£102bn) and $200bn (£164bn), hidden away in a network of shell companies and the accounts of friends.
Mr Plekhov has been sanctioned by the UK government, and Mr Roldugin has also been sanctioned by the UK, the EU and the US, which described him as a "custodian of President Putin's offshore wealth".
The Cyprus connection
The Cyprus Confidential investigation is based on 3.6 million confidential corporate records from companies providing offshore services in Cyprus, and has focused on its close financial relationship with Russia and now-sanctioned oligarchs, many of whom have used the island to manage their secret offshore holdings.
They include documents from a corporate service provider in Cyprus called MeritServus, originally obtained by the whistleblowing group Distributed Denial of Secrets. MeritServus was itself sanctioned by the UK earlier this year, after internal documents revealed it had breached sanctions on behalf of one of its Russian clients.
MeritServus also worked with Mr Abramovich's companies in Cyprus. The oligarch's wealth totals more than $9bn (£7.3bn) and he has made numerous public investments in sports, arts and high-value properties. He became one of the best-known and influential Russian oligarchs in the UK after buying Chelsea FC in London in 2003.
He has downplayed his relationship with President Putin, and challenged suggestions of a close financial relationship or that he has acted on behalf of the Russian leader.
In 2010, a spokesperson for Mr Abramovich said he had "no financial relationship of any kind with [then] Prime Minister Putin".
And in 2021 he sued journalist Catherine Belton over a passage in her book, Putin's People, referring to evidence alleging that he had purchased Chelsea FC in 2003 at President Putin's behest. The case was settled out of court with an agreement by the publisher "to record the position more accurately" and add "a more detailed explanation of Mr Abramovich's motivations".
The UK and EU placed Mr Abramovich under sanctions in March 2022 following the Russian invasion of Ukraine.
The EU said: "He has had privileged access to the president, and has maintained very good relations with him. This connection with the Russian leader helped him to maintain his considerable wealth."
Mr Abramovich challenged the EU sanctions in court earlier this year. His lawyer claimed the restrictions were prompted by the Russian businessman's "celebrity" rather than "based on evidence".
But the secret deal with Mr Roldugin and Mr Plekhov suggests a close financial relationship between Mr Abramovich and President Putin.
"This case obviously puts more information onto the table and further endorses the alleged connection between Putin and Abramovich in a way that becomes increasingly difficult to deflect," says Tom Keatinge, director of the Centre for Financial Crime and Security Studies at the defence think tank RUSI.
The deal
A complex web of companies in Cyprus and the British Virgin Islands, and a trust, concealed the football tycoon's involvement in the transaction - until now.
Leaked documents reveal the former Chelsea boss's relationship with two companies that bought a combined 25% stake in Video International in 2003.
The two companies - Finoto Holdings and Grosora Holdings - were created in early 2003. Both were ultimately owned, through a series of shell companies, by the Sara Trust Settlement - a trust of which Abramovich was an ultimate beneficiary.
Each company bought a 12.5% stake in the Russian advertising giant in September 2003 for the same price - about $130,000 (£80,000) each.
The price paid was "ridiculous", says Vladimir Milov, a former energy minister in President Putin's first term and now a vocal opposition leader. "That stake was clearly worth much more, by many orders of magnitude."
At the time of the purchase, Video International enjoyed a dominant position in the domestic TV advertising market, taking a cut of any advertising airtime purchased on Russian channels.
The company was "half a step away from the Kremlin administration", according to Mr Milov.
Mr Abramovich had a stake in Video International for the next seven years. At one point, the company declared a turnover of "more than $2bn [£1.29bn]". Dividends of $30m (£19.3m) were paid out to Finoto and Grosora over that period.
Video International reported revenues of $3bn (£1.9bn) in 2010. However, Finoto and Grosora each sold their investment that year for just $20m (£19.5m), a price that appears to be below its fair market value.
Finoto Holdings sold its stake to Med Media Network, a company nominally owned by Sergei Roldugin.
On the same day as the Finoto Holdings sale, the other Abramovich-linked company, Grosora Holdings, sold its 12.5% stake to Namiral Trading Ltd, a company later linked to Aleksandr Plekhov.
Putin's 'wallets'
Financial links between President Putin and Mr Roldugin were uncovered in 2016 as part of the Panama Papers, which involved the leak of millions of confidential documents from the Panama-based law firm Mossack Fonseca.
Mr Roldugin, along with Mr Plekhov, was at the centre of a suspected money-laundering scheme run by Bank Rossiya and some of President Putin's closest associates. Bank Rossiya was sanctioned by the US government in 2014, which described it as "the personal bank for senior officials of the Russian Federation".
Mr Roldugin told the New York Times at that time that he was not a businessman and did not "have millions". However, at least on paper, he appeared to have an offshore fortune of over $100m (£61m).
"Rodulgin clearly serves… as a cover-up for Putin's personal beneficial ownership," says Vladimir Milov. "This guy is absolutely clearly 100% a nominal figure because he does not understand anything about business, finance, international transactions and so on."
Revelations in the Panama Papers about bank accounts held by Mr Roldugin in Switzerland, led to an investigation and the trial of four Gazprombank employees earlier this year. The bankers were accused by Swiss prosecutors of failing to properly check accounts opened in the name of Roldugin.
They were also said to have failed to identify the Russian president's friend as politically exposed - someone whose position or relationships mean that they may be more exposed to risks of corruption, and require more checks under international finance regulations.
According to the indictment, accounts with Gazprombank had been simultaneously established for both Med Media Network and Namiral Trading Ltd with an identical "purpose and structure" to "hold shares and receive dividends" from Video International.
The prosecutors said the arrangement represented a direct extension of "assets managed... for the Russian political establishment".
Mr Roldugin and Mr Plekhov were "straw men", and not the real beneficiaries of the accounts, the prosecutors alleged.
All four bankers were convicted, but are reported to be appealing.
The BBC wrote to Mr Plekhov, Mr Roldugin, Bank Rossiya and President Putin for comment but have received no response.
'Secret conduit'
Many wealthy Russians have used Cyprus, an EU member state, as part of their network of offshore investments. Through these economic relations, Russia is "worth tens of billions of dollars to the Cyprus economy each year", says Fergus Shiel of the ICIJ.
The Cyprus Confidential investigation raises "grave issues" for European institutions and EU member states, Shiel continues. "What we can see in these documents is a European member being a conduit for the secret financial operations of the Kremlin, of Vladimir Putin and his cronies."
However, there are signs that Cyprus may be cleaning up its act.
Following the invasion of Ukraine in 2022, many Russians considered close to the financial assets of President Putin were sanctioned by the EU. This has had direct consequences for those with Cypriot investments.
"The sanctions brought home that Cyprus cannot be used by oligarchs to support the dirty orders of Putin," says Alexandra Attalides, an independent Cypriot MP.
Meanwhile, reports suggest Mr Abramovich now spends his time between the Russian resort of Sochi, Istanbul and Tel Aviv. He has Russian, Israeli and also Portuguese passports.
The oligarch remains the subject of sanctions in the UK and EU, but not in the US, where he is understood to still hold considerable assets.