Police are not naming the man because they have not yet formally identified him, but he is believed to be Nikolai Glushkov – a friend of Boris Beresovsky, the Russian oligarch who criticised President Vladimir Putin.
Officers say the investigation is a precaution.
Sky News’ home affairs correspondent Mark White said Mr Glushkov’s home in New Malden, south west London, is being treated as a crime scene.
In a statement, Scotland Yard said: “Police were called by the London Ambulance Service at 10.46pm on Monday, 12 March, to reports of a man found deceased at a residential address in Clarence Avenue, New Malden.”
Mr Glushkov’s family are being looked after by specialist police officers.