The death is believed to be of Nikolai Glushkov, who was convicted of fraud and money-laundering in Russian Federation and served five years in prison before being released and fleeing to the UK.
Nikolai Glushkov, who received political asylum in the United Kingdom, was found dead at his London home on Monday by friends and family.
Sky News' home affairs correspondent, Mark White, said Mr Glushkov's home in New Malden, southwest London, is being treated as a crime scene.
Police were called at 2246 GMT (6.46 p.m. ET) on Monday to a residential address in Clarence Avenue, New Malden, where a man in his 60s was found dead.
Police said the death was not linked to the poisoning of Sergei Skripal and his daughter in Salisbury. He was known as the "godfather of the Kremlin" before fleeing to London in 2000 after a row with Russian leader Vladimir Putin.
Counter-terrorism police were leading the investigation "because of associations that the man is believed to have had".
Glushkov's death has done nothing to play down fears amid a highly charged political climate with commentators quickly linking it to Salisbury.
"The death is now being treated as unexplained", he said.
The 68-year-old was a close associate of the late tycoon Boris Berezovsky, who died at his ex-wife's Berkshire home in 2013.
Glushkov had worked for Russian airline Aeroflot and at Berezovksy's LogoVAZ vehicle company.
In 2017, during a trial in absentia in Russia, Glushkov was sentenced to eight years in prison for stealing $123m from Aeroflot.
Mr Berezovsky was himself found dead on a bathroom floor at his home in southern England in 2013 with a scarf around his neck.
Glushkov had said at the time that he did not believe Berezovsky died of natural causes.