MOSCOW -- Boris A. Berezovsky, once the richest and most powerful of the so-called oligarchs who dominated post-Soviet Russia, and a close ally of Boris N. Yeltsin who helped install Vladimir V. Putin as president but later exiled himself to London after a bitter falling out with the Kremlin, died Saturday.
He was 67 and lived near London, where last year he lost what was billed as the world's largest private lawsuit in history -- an epic tug-of-war over $5.1 billion with another Russian oligarch, Roman A. Abramovich, in which legal and other costs were estimated to be about $250 million.
Mr. Berezovsky's death was first reported in a post on Facebook by his son-in-law Egor Schuppe, and confirmed by Alexander Dobrovinsky, a lawyer who had represented him.
Mr. Dobrovinsky, who runs a law firm with offices in Moscow and London, wrote in Russian on his own Facebook page: "Just got a call from London. Boris Berezovsky has committed suicide. The man was complex. An act of desperation? Impossible to live poor? A series of blows? I am afraid that no one will know the truth."
A spokesman for Mr. Putin said that Mr. Berezovsky had recently sent a letter asking the president for forgiveness and permission to return to Russia. "Some time ago, maybe a couple of months, Berezovsky sent Vladimir Putin a letter, written by himself, in which he admitted that he had made a lot of mistakes," the spokesman, Dmitri S. Peskov, said on the Russia 24 television channel. "He asked Putin for forgiveness for the errors to be able to return home."
Mr. Peskov said he did not know Mr. Putin's reaction, but that "news of anyone's death, no matter what kind of person they were, cannot arouse any positive emotions."The Thames Valley police in Berkshire, England, about an hour outside London, issued a statement on Saturday saying that they were investigating the "unexplained" death of a 67-year-old man in Ascot, a posh town near one of Britain's most storied racetracks and one of its premier golf courses, Royal Ascot.
The police statement did not name Mr. Berezovsky, but British news reports said an investigation was under way at his home.
In London, Mr. Berezovsky had adopted much the same style as an oligarch back home in Russia. He was driven in chauffeured cars, sometimes a Maybach sedan, that were typically followed by a Range Rover bearing dark-suited bodyguards.
But recent news reports said Mr. Berezovsky had begun to sell personal assets, including a yacht and a painting by Andy Warhol, "Red Lenin," to pay debts related to the lawsuit.
The lawsuit, in which Mr. Berezovsky brought a claim against Mr. Abramovich in a dispute over the sale of shares in Sibneft, a Russian oil company, and other assets, ended in a spectacular defeat.
In her ruling, the judge in the case, Elizabeth Gloster, called Mr. Berezovsky an "unimpressive and inherently unreliable witness" and at times a dishonest one. By contrast, the judge said Mr. Abramovich had been "a truthful, and on the whole reliable, witness." Mr. Berezovsky's legal troubles worsened in recent weeks with a claim by his former girlfriend, Elena Gorbunova, that he owed her about $8 million from the sale of a house they owned in Surrey, England. Mr. Berezovsky was a Soviet mathematician who after the fall of Communism went into business and figured out how to skim profits off what was then Russian's largest state-owned carmaker. Along with spectacular wealth, he accumulated enormous political influence, becoming a close ally of Mr. Yeltsin's.
With Mr. Yeltsin's political career fading, Mr. Berezovsky helped engineer the rise of Mr. Putin, an obscure former K.G.B. agent and onetime aide to the mayor of St. Petersburg, who became president of Russia in 2000 and last May returned to the presidency for a third term.
After his election, Mr. Putin began a campaign of tax claims against a group of rich and powerful Russians, including Mr. Berezovsky and Mikhail B. Khodorkovsky, the oil tycoon, who remains jailed in Russia.
Mr. Berezovsky fled to London, where he eventually won political asylum and at one point raised tensions by calling for a coup against Mr. Putin.
David E. Hoffman, the author of "The Oligarchs: Wealth and Power in the New Russia," an in-depth exploration of the role of such magnates in the era after the breakup of the Soviet Union, said Mr. Berezovsky stood out for seeking not only wealth but also political clout.
"Boris Berezovsky was among that wave of oligarchs who realized that great fortunes were to be made in the massive sell-off of assets in the new Russia," Mr. Hoffman said by e-mail on Saturday. "While many of his peers also saw the opportunity, Berezovsky was more focused than most on the role that politics would play. He realized the need to co-opt those in power in order to make deals. He did it from the early days with automobiles and later with oil. He understood the bond of wealth and power. He was not always an astute judge of politics in Russia's young democracy, but he grasped that power was essential to seizing the enormous wealth at hand."
Mr. Berezovsky had an outsize, if hardly always benevolent, role in post-Soviet Russia.
George Soros, a financier and a critic of the Russian oligarchs, had likened them to 19th-century American robber barons. But if that was an apt metaphor, the power and influence of these new tycoons was amplified by the legal and political vacuum left by the collapse of the Soviet Union.
Mr. Berezovsky amassed his fortune at first in automobiles, including a business he formed in 1993 with Aleksandr Voloshin, who would later become Mr. Yeltsin's chief of staff. But like other oligarchs, Mr. Berezovsky's interests spread across many sectors of the post-Soviet Russian economy, to oil; media, including control of the country's main television station; and Aeroflot, the Russian airline.
He survived an assassination attempt in 1994, a car bombing in which his driver was killed.
The assassination attempt connected him to a K.G.B. officer, Alexander V. Litvinenko, who was poisoned by the radioactive isotope polonium 210 in London in November 2006.
Mr. Litvinenko, then working for the F.S.B., the domestic successor to the K.G.B., was assigned to investigate the bombing, and Mr. Berezovsky became his close mentor and later employer.
Mr. Berezovsky helped Mr. Litvinenko flee Russia in 2000 before he, too, left the country to seek asylum in London.
On the day he was poisoned, Nov. 1, 2006, Mr. Litvinenko went from a meeting with several Russians at a four-star hotel in central London to Mr. Berezovsky's nearby office. There he met with a Chechen exile, Akhmad Zakayev, another Berezovsky protégé, and the two drove together to adjacent homes financed by Mr. Berezovsky, in North London.
After Mr. Litvinenko's death, and with his once fabulous wealth dwindling during his time in London, Mr. Berezovsky slowly withdrew his financial support for Mr. Litvinenko's widow as she pressed for an inquest into his death, now scheduled to begin in May.
Boris Abramovich Berezovsky was born in Moscow on Jan. 23, 1946, to Abram Berezovsky, a civil engineer who worked in construction, and Anna Gelman, at a time when the Soviet Union was still recovering from the Second World War.
He studied forestry and mathematics and graduated from the Moscow Forestry Engineering Institute. He worked as an engineer and researcher until the late 1980s.
At one point in the mid-1990s, Mr. Berezovsky served on Russia's security council, only to be dismissed from that post by Mr. Yeltsin in 1997. Six months later, Mr. Berezovsky failed in a bid to prevent Mr. Yeltsin from naming Sergei V. Kiriyenko as prime minister. Shortly afterward though, Mr. Yeltsin appointed Mr. Berezovsky as chief executive of the Commonwealth of Independent States. It was the sort of surprise move that Mr. Yeltsin relished but part of a pattern that even his supporters later came to view as erratic and ultimately contributed to his resignation.
Mr. Berezovsky and Mr. Putin had been close, and Mr. Berezovsky aided Mr. Putin's rise to the presidency. But signs came quickly that Mr. Berezovsky had fallen out of favor of the leader. In October 2000, just 10 months after Mr. Yeltsin's resignation, Mr. Berezovsky was stripped of some of the trappings of his prior status -- ordered to vacate a spacious government country house and to return the government plates on his limousine.
In March 2003, the British authorities arrested Mr. Berezovsky and said they were beginning a process that could lead to his extradition. But he was granted political asylum later that year apparently after a determination that Russia was seeking him solely on political grounds.
The Kremlin had declared Mr. Berezovsky a wanted man, and had unsuccessfully sought his extradition. In 2007, he was convicted of fraud charges by a Russian court in absentia and sentenced to six years in prison, and had potentially faced prosecution in at least 10 other cases.
The sharpest blow to his wealth came from the failed lawsuit against Mr. Abramovich. Mr. Berezovsky had accused Mr. Abramovich, among other things, of a scheme that forced him to sell his stake in Sibneft for a fraction of its true value.
On the day last August when the court ruled against him, Mr. Berezovsky attempted an air of nonchalance.
"Life is life," he said, flanked by bodyguards, before driving off in a chauffeur-driven Mercedes.
Andrew E. Kramer contributed reporting from Moscow, Alan Cowell from Venice and Ravi Somaiya from New York.
Correction: March 23, 2013, Saturday
This article has been revised to reflect the following correction: A previous version of this article said Boris Berezovsky died at his home in London. His home was outside of London.
This article originally appeared in The New York Times.