One of Roman Abramovich's closest associates misled a UK court to help secure ownership of Siberian oil fields, leaked documents seen by the BBC suggest.
Eugene Tenenbaum, a former director of Chelsea Football Club, helped to manage the billionaire's wealth at the time of the case in 2008, which Abramovich won.
Tenenbaum was giving evidence during proceedings taken against Abramovich by a company owned by another Russian billionaire, who was claiming the oil fields had been stolen from him and other investors.
The leaked documents now suggest there was a premeditated attempt by Abramovich to reduce the value of the other investors' stake in the Siberian venture.
Roman Abramovich's lawyers say their "client denies all the allegations made against him".
Lawyers for Eugene Tenenbaum say he "denies in the strongest terms any suggestion that he committed perjury or contempt of court".
The 2008 court case centred on the ownership of Siberian oil fields being developed jointly between Abramovich's company, Sibneft, and the Russian subsidiary of the British company, Sibir Energy PLC, owned by Russian billionaire Chalva Tchigirinsky.
Sibneft contributed money, while the subsidiary Yugraneft provided the licences for the oil fields.
Their partnership - which became known as Sibneft-Yugra - ended acrimoniously.
Sibir's 50% interest in the joint venture had been diluted, or reduced, to less than 1%. It said this had been done without its knowledge.
Unsuccessful attempts to sue Sibneft in both Russia and the British Virgin Islands led to the court proceedings in London.
At court in 2008, Yugraneft claimed "to have been the victim of a massive fraud directed by Mr Abramovich", as a result of which it had suffered a loss of "not less that $2 billion", which in 2008 was worth about £1bn.
But Abramovich's team claimed that Yugraneft had been unable to repay loans of more than $30m, so lost its right over the shares.
Abramovich won the case on a number of fronts, including that Yugraneft's case had been an abuse of process.
However, Panorama has uncovered new evidence suggesting the court was misled, in a joint investigation with the Bureau of Investigative Journalism, external.
The evidence is contained in a leak of documents about Abramovich's finances and the operations of a network of offshore companies administered from Cyprus between 1995 and 2022. It raises serious questions about the integrity of some of the oligarch's professional advisers.
We have already found that the world's biggest accountancy firm, Deloitte, helped in falsifying documents to hide his wealth and evade tax.
The BBC and media partners including the Guardian, Paper Trail Media, the Organised Crime and Corruption Reporting Project and the Cyprus Investigative Reporting Network - as well the Bureau of Investigative Journalism - have been reporting on the leaked files since 2023 as part of the International Consortium of Investigative Journalists' Cyprus Confidential investigation, external.
In the High Court, lawyers for Yugraneft argued the case should be heard in London, and that Tenenbaum, who was based there at the time, must have been aware of the dilution of Yugraneft's interest.
But Tenenbaum denied this and told the court "he only became aware of Sibneft-Yugra in Spring 2004".
He also testified he had "searched" his "files, emails and documents from 2002 to 2004" and had in his "possession no records or documents or other communications that relate to Sibneft-Yugra".
But the leaked documents suggest he misled the court.
They show that in October 2003, months before Tenenbaum said he first became aware of Sibneft-Yugra, he had authorised Abramovich's purchase of shares in the joint venture.
Two months later, Tenenbaum authorised another financial deal related to Sibneft-Yugra.
Andrew Mitchell KC, an expert in financial services law, told Panorama that it would be "very serious" if Tenenbaum's 2008 evidence was false and that it could "range from being perjury, to an attempt to pervert the course of justice, to being a contempt of the court".
Tenenbaum's lawyers told us "he gave his evidence honestly and in good faith" and that he "cannot recall the context or detail of documents" more than 20 years old.
The judge made clear in his 2008 judgement that Tenenbaum's knowledge of the Sibneft-Yugra deal would not have made a difference to his decision, however the leak also suggests some of the documents presented by Abramovich's legal team to the High Court had been falsified.
One of the agreements was dated August 2003, but appears not to have been created or actually signed until 2007.
Abramovich's team had argued in court that Yugraneft's interest had been diluted because of the need for finance for the joint venture.
They claimed Tchigirinsky had approached them "in the spring of 2002" with a view to new financing arrangements. Tchigirinsky denied this was the case.
The leaked documents show that before then, in January 2002, Abramovich's Russian associates had already prepared a bundle of documents.
They included agreements under which two Abramovich companies would acquire shares in Sibneft-Yugra, increasing his stake and diluting Yugraneft's.
This evidence suggests they were already planning to dilute Yugraneft's interest and that the argument they used in court about financing was a smokescreen.
Lawyers for Abramovich said: "The Judgment made no finding that the dilution was premeditated", or that it was "fraudulent". They added: "The Judge found that our client received no property belonging to Yugraneft, and that no Yugraneft asset could be traced into his hands."