It is proving to be a very modern divorce. Armies of lawyers and advisers; hundreds of millions of pounds at stake; priceless art; a superyacht; a key lieutenant switching sides; the son dragged into the proceedings by his mother. No wonder some involved have likened it to The War of the Roses, the dark Hollywood comedy about a feuding couple starring Kathleen Turner and Michael Douglas. But now attempts to secure the assets awarded following Britain’s biggest, bitterest marital breakup may hinge on how the high court views an arcane financial practice dating back to feudal times. The marital dispute between Farkhad Akhmedov, a Russian citizen and ally of President Putin, and his wife, Tatiana Akhmedova, will have even learned property lawyers dusting off ancient textbooks as they argue over whether third parties can finance another person’s legal claim, a practice that is illegal in some countries. The pair have been at loggerheads since December 2016 when the high court awarded Tatiana Akhmedova, 52, a record £453m, representing a 41.5% share of her husband’s marital assets. He had claimed that documents prove the couple were divorced 20 years ago in Moscow and therefore his ex-wife’s attempts to seize his assets – which include works by Rothko, Warhol and Hirst, not to mention a $450m superyacht once owned by Roman Abramovich – were fraudulent. But the high court said it could find no evidence of the earlier divorce and so the decades-long battle continues. Now Akhmedova, will return to the high court tomorrow to have Temur, the couple’s eldest son, who lives in London, joined to the case. Her lawyers want private emails between father and son disclosed in court. Adding to the acrimony is the fact that one of Akhmedov’s former financial advisers went to work for his ex-wife after being sacked. Akhmedova is backed by Burford Capital, a litigation finance firm that will take a slice of her payout if it succeeds in recovering the £453m the high court awarded its client. But lawyers for Temur, 26, will argue that his mother’s claim is unlawful because it is being funded by a third party for financial gain. The practice, known as champerty, dates back to feudal times, when noblemen would lend their high-profile support to legal claims in return for a share of any successful claim to recovered property. Temur’s lawyers, who have applied for an order to prevent his key personal financial information from being revealed, will ask the court to force his mother to disclose the commercial arrangement she has with Burford and to argue that third-party financing in matrimonial matters is not permissible. According to papers filed into court by Temur’s legal team, such claims run contrary to English law and to the Matrimonial Causes Act of 1973. The Observer also understands they will point to a recent legal judgment handed down in Hong Kong which ruled against champerty. “Temur has had to put up with the upsets of his parents’ divorce since he was seven years old,” a friend said. “He is aware that many genuine efforts have been made to resolve matters between them. But the presence of Burford’s financial investment in the case has blocked these attempts.” It has been previously reported that Farkhad Akhmedov, 64, once offered his ex-wife an £80m lump sum when he dies, £4m a year for life and and some prized antiques which included Napoleon’s old writing desk, worth an estimated £30m. As a funded claimant, Akhmedova retains control of all litigation decision-making and settlement. But Akhmedov’s representatives claim she cannot settle even if she now wished as Burford needs to obtain a full settlement to recoup its costs. In response to specific questions, lawyers for Akhmedova said: “The proceedings in this matter are ongoing and we do not intend to comment in advance of the forthcoming hearing.” They said that it was a matter of public record that the high court found documents relied on by Akhmedov in support of his claims that the couple were divorced in Russia in 2000 were “at all material times, forged”. They added: “It is also a matter of public record – widely reported in the media – that Ms Akhmedova is pursuing Temur for his part in Mr Akhmedov’s evasion of the 2016 judgment and financial award in her favour.”