Boris Berezovsky, the Russian oligarch who helped broker Vladimir Putin's rise to the Kremlin's top job only to become his fierce enemy, has been found dead at his home in Britain in unclear circumstances. He was 67.
"His death is currently being treated as unexplained and a full inquiry is under way," police said in a statement on Saturday that did not identify Berezovsky by name.
One of Putin's fiercest critics, Berezovsky spoke openly of overthrowing the Russian leader and said he feared for his life though it was unclear how the mathematician-turned-tycoon had died.
Russian media said he had been found dead in a Russian sauna at his house and that he may have committed suicide or had a heart attack, though the British police said only that they had launched an investigation into an unexplained death.
"I can confirm he died in his home. I've known him for a long, long time, we have spent a lot of time together," Andrei Sidelnikov, a Russian dissident living in London who was a friend of Berezovsky's, told Reuters.
"I am shocked. It is the end of an epoch."
Police said they had launched a full investigation into the death and cordoned off his home in the commuter town of Ascot near London
Once one of Russia's richest men, Berezovsky fell out with the Kremlin under Putin and fled Moscow for Britain where he became a vociferous opponent of Russia's most powerful man.
But the businessman once cast as the 'godfather of the Kremlin' was humiliated in 2012 when he lost a $6 billion legal battle with Chelsea Football Club owner Roman Abramovich in London.
The judge, Elizabeth Gloster, said Berezovsky was an "unimpressive and inherently unreliable witness" who gave sometimes dishonest evidence and would say "almost anything to support his case".
Once an oligarch who could open every door in the Kremlin, some associates said Berezovsky had grappled with the financial impact of losing the case, which lawyers at the time said could open him up to claims for costs of more than $100 million.
He was also a close friend of Alexander Litvinenko, whose death from radioactive polonium poisoning in London in 2006 plunged relations between Russia and Britain to a post Cold War low.
Berezovsky made his money after setting up a chain of car dealerships in 1989 but then turned his money towards the real prize in Russia: the natural resources of a former superpower.
Leveraging his political ties with the family of Boris Yeltsin, Berezovsky became one of the most powerful tycoons in the corrupt economy which emerged from the ruins of the Soviet economy, gaining stakes in oil and metals companies.
He orchestrated the re-election of Boris Yeltsin in 1996, participated in the privatizations of the 1990s and then helped push Putin towards the presidency.
Putin's spokesman, Dmitry Peskov, told state-run Rossya-24 television that Berezovsky had written Putin and asked for help in returning to Russia.
"Some time ago, maybe a couple of months ago, Berezovsky sent Vladimir Putin a letter he wrote personally, in which he acknowledged that he had made many mistakes, asked Putin's forgiveness for these mistakes and appealed to Putin to help him return to his homeland," Rossiya-24 quoted Peskov as saying.
A London-based spokeswoman for Berezovsky said she was checking the reports of his death in his home in the county of Berkshire. One of his London lawyers declined to comment.
Litvinenko's widow Marina, a close personal friend of Berezovsky, told Reuters: "It's very very hard for me and for my family.
"He was not just a friend or a famous person, he was more than that for me," she said, fighting back tears.
She refused to give any details surrounding his death.