His so-far unexplained death could add a further dimension to UK-Russian relations, which are under considerable strain after the poisoning of the 66-year-old Skripal with a deadly nerve agent.
Home Secretary Amber Rudd said MI5 will look at allegations of foul play in a string of fatalities that have been linked to Moscow by USA intelligence. He conceded that in the period before his friend's death they had quarrelled.
British Prime Minister Theresa May told parliament on Monday that it is "highly likely" that Russian Federation is responsible for the nerve agent attack in Salisbury. But the Russian embassy said it would not respond to "London's ultimatum" unless it is given access to samples of the substance, and the country's foreign ministry spokeswoman Maria Zakharova warned Britain "should not threaten a nuclear power".
Attorney Andrei Borovkov told Russian media outlets that Glushkov had died, but said he was unaware of the time and circumstances.
Mr Glushkov, who first met Mr Berezovsky in 1989, always believed that he had his friend had been killed.
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Police and MI5 are expected to examine allegations that a series of deaths on United Kingdom soil, including the Berezovsky case, may be linked to Russian Federation. The reliability of Mr Glushkov's evidence was also called into question.
In court, Berezovsky claimed he and Abramovich had been partners in the 1990s in an oil firm, Sibneft, and accused the Chelsea football club owner of cheating him out of $5bn (£3.2bn).
"The death is now being treated as an unexplained".
Russian Nikolai Glushkov, 68, who was associated with a leading critic of Vladamir Putin has been found dead in London, UK. Police said a post-mortem examination showed no sign of a violent struggle, and an inquest recorded an open verdict. "[But] I don't believe it was suicide", he said.
Berezovsky was found dead in 2013 on the bathroom floor of his home in Berkshire, west of London. He knew Alexander Litvinenko, who was killed in 2006 by two Kremlin assassins using a radioactive cup of tea.
Glushkov was found dead late on Monday at a house in southwest London's New Malden area. Officers said there was no evidence to link it with events in Salisbury and the assassination attempt of former double agent Sergei Skripal.