The pair were killed because of a failed multi-million-pound Russian property venture dubbed Project Moscow, claims former offshore accountant Paul Blanchard. Mr Blanchard, who went on to become a police informant, says the same Bulgarian assassination team were responsible for the unsolved murder of Brinks-Mat gangster John “Goldfinger” Palmer, because of Russian organised crime involvement in Tenerife timeshares.

       Berezovsky, 67, a vocal critic of Vladimir Putin, was found in 2013 with a ligature around his neck in the Surrey mansion he had bought from Young, with the subsequent inquest recording an open verdict.

       Surrey Police had found no evidence of third-party involvement, and the coroner said: “I am not -saying Mr Berezovsky took his own life, I am not saying Mr Berezovsky was unlawfully killed. What I am saying is that the burden of proof sets such a high standard it is impossible for me to say.”

       Young, 52, who is said to have helped Berezovsky launder money, died after plunging on to railings below his ￡3m London flat in 2014, after telling police and friends -several times that hitmen with links to Moscow wanted him dead.

       He had earlier been declared bankrupt following Russian property investment deals. The Met Police ruled it a non--suspicious death. So too did an inquest in July 2015 that heard Young was suffering with mental health problems. It found there was insufficient evidence for a suicide verdict.

       There have long been suspicions the pair were murdered, with theories their deaths were made to look like suicides. A 2018 Met Police and MI5 review of the cases and 12 other deaths of people connected to Russia, carried out in the wake of the Novichok attacks on Sergei and Yulia Skripal in 2018, went on to conclude that they were all non-suspicious.

       Essex Police continues to investigate Palmer’s murder, after he was shot dead in the garden of his Brentwood home in June 2015. They believe his involvement in timeshare fraud and an upcoming Spanish prosecution was a likely motive for his killing. But in his new book Wanted: Secrets of a Mafia Whistleblower, Mr Blanchard claims that all three men were murdered by the same team.

       Mr Blanchard, 76, from York, details how he became an offshore accountant in Marbella and Tenerife, to set up tax avoidance schemes for the wealthy. He claims this led to him “inadvertently” setting up businesses and accounts for people linked to alleged Mafia networks across the globe.... before he turned whistleblower for the Spanish authorities after spotting potential huge frauds.