Sergei Skripal was poisoned along with his daughter.
Police are treating the death as unexplained and have put counterterrorism detectives in charge of the case.
The counter-terrorism command unit is leading the investigation into the March 12 death of Nikolai Glushkov, a close associate of late Russian tycoon Boris Berezovsky, the Metropolitan police said.
Andrei Borovkov told Russian media outlets on March 13 that his client, Nikolai Glushkov, has died, but said he was unaware of the time and circumstances.
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But there was "no evidence", it said, "to suggest a link to the incident in Salisbury", adding that the death is being treated as "unexplained".
Police officers man a cordon near where Sergei Skripal was poisoned along with his daughter.
The Met said officers believe they know who the man is, but he is yet to be formally identified.
Berezovsky was found hanged at his home and an inquest failed to determine whether he had killed himself or died from foul play.
Glushkov held a top position with Russia's state airline Aeroflot and worked under Berezovsky in the nineties.
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He was arrested in 1999 and put on trial for embezzling US$7 million from Aeroflot.
Berezovsky was found dead in March 2013 with a scarf tied around his neck in the bathroom of a luxury mansion in Ascot, an affluent English town near the Queen's Windsor Castle, west of London.
Glushkov came to the United Kingdom after being freed from prison and spoke out against Berezovsky's death claiming he never believed his friend died of natural causes. In 2004, he was sentenced to three years and three months behind bars but was released in the courtroom having served the term in custody.
In March last year, he was sentenced to eight years in prison in absentia and issued a one million Russian Ruble fine for allegedly defrauding Aeroflot of $122 million during his tenure as finance director there in the late 1990s.
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