There, he gave evidence in court against Roman Abramovich, another Russian oligarch who remains close to the Kremlin.
British police confirmed the death of "a man" at a residential address in Clarence Avenue, New Malden in London, and his family members were informed, giving no details of the deceased person's identity. It did not identify him by name.
Nikolai Glushkov, a prominent businessman whose associates include a prominent critic of the Kremlin who died in 2013, was found dead in his home on Tuesday, according to his lawyer, Andrei Borovkov.
A subsequent inquest failed to determine whether he had killed himself or died from foul play.
The death of a 68-year-old Russian man in London is being investigated by counter-terrorism officers, Scotland Yard has said.
There is no evidence to suggest a link to the March 4 poisoning of former spy Sergei Skripal and his daughter, the statement said.
Mr Glushkov was the former deputy director of airline Aeroflot and a former finance manager of ActoVAZ.
Jamie Carragher suspended from Sky Sports duties after spitting incident
Carragher wrote: "As usual, there have been a number of people who have said, 'It is the worst thing that can happen on a pitch". All manner of football personalities have reacted with the likes of Danny Murphy , Vinnie Jones and Joey Barton wading in.
Glushkov received political asylum in Britain after serving five years in prison in Russian Federation for money laundering and fraud, the Guardian newspaper reported. He spent five years in jail and was freed in 2004.
Russian media said he was granted political asylum in Britain in 2010. Berezovsky had been an early ally of Putin - but their relationship changed drastically, culminating in the tycoon leaving Russian Federation and being locked in a multi-billion-dollar legal battle with another oligarch, Roman Abramovich, over proceeds from the Sibneft oil company.
Mr Berezovsky, who became Mr Putin's most strident critic from overseas, was found dead in 2013 - police concluded it was suicide but a coroner recorded an open verdict.
Glushkov was a friend of Berezovsky and argued publicly that he believed the exiled tycoon had been murdered.
He added: "Too many deaths [of Russian emigres] have been happening". The cause of death is still unknown.
"You have the deaths of Boris and Badri over a short period of time".
But his case was revived by a Moscow court last year, which sentenced him in absentia to eight years for allegedly embezzling more than $122m from Aeroflot.
May gave Russian Federation until the end of Tuesday to respond to her allegations.