Nikolai Glushkov's death comes amid heightened concern following a nerve agent attack on a former Russian spy in England and as the government said it would investigate the deaths of a number of Russia-linked individuals which have raised suspicions.
Glushkov, who worked for Berezovsky's companies Aeroflot and AvtoVAZ, was discovered by his daughter Natalya at his house in New Malden in southwest London on Monday, the reports said.
Russia's Kommersant newspaper quoted Glushkov's family saying that the body carried "traces of strangulation".
It said "it was not yet clear whether it was murder or suicide".
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Britain's counter-terrorism police, which is leading an investigation into the attack in Salisbury on former double agent Sergei Skripal, said it was investigating the death of a man in his 60s in Kingston in southwest London but did not identify him.
Counter-terror police said it was in charge of the probe "as a precaution because of associations that the man is believed to have had", a spokesman said.
But he added that there was "no evidence to suggest a link to the incident in Salisbury".
"The death is currently being treated as unexplained," he said.
Glushkov received political asylum in Britain after serving five years in prison in Russia for money laundering and fraud, the Guardian newspaper reported.
Berezovsky, a powerful oligarch and one-time supporter of Putin who turned against his former protege, was found hanged in a bathroom at his home outside London in 2013.
Glushkov had said at the time that he did not believe Berezovsky died of natural causes.
"I'm definite Boris was killed," he was quoted by the Guardian as saying at the time.
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