The 68-year-old Nikolai Glushkov was found dead in his home in New Malden on March 12, of unexplained causes, and the police pathologist has just concluded that his previously unexplained death was due to "compression of the neck".
The pathologist's conclusion means a murder inquiry has been officially opened.
Glushkov was one of a group of Russian exiles living abroad to avoid the ire of President Vladimir Putin, and a close friend of prominent Kremlin critic, the late Boris Berezovsky. His death comes as the British authorities are engaged in a firey diplomatic spat with Russia, accusing Moscow of attempting to murdering a former spy and his daughter with a nerve agent in Salisbury.
Glushkov also once testified against billionaire Roman Abramovich, a Putin ally who is also the owner of the Chelsea Football Club, in a court case.
Police stress that for the moment they have no evidence linking the two incidents.