Sim cards 'laid out'
Scotland Yard opened a murder inquiry on Wednesday, two days after the body was discovered in an apartment in Pimlico, 1.6km from MI6 headquarters.
Police entered the residence on Monday afternoon following reports that the occupant, who was said to have been in his 30s, had not been seen for some time.
The Daily Mail reported that police officers found the man's mobile phone and a collection of sim cards "carefully laid out" inside the property.
Scotland Yard refused to say whether there were signs of a struggle or how
the man may have been killed, but said suicide had been ruled out.
Paul Brennan, Al Jazeera's correspondent in London, said the man could have been dead for up to two weeks.
"A post-mortem is taking place today, at which the cause of death will be ascertained, it is hoped. But a stabbing appears to be the version that is coming from police sources at the moment."
He said the fact the death was being investigated by the homocide squad rather than an anti-terror or special branch of the police suggested the murder may not be directly related to espionage.
A police source told the Press Association: "The suggestion there is terrorism or national security links to this case is pretty low down the list of probabilities."
'Den of spy activity'
If reports of the victim being a spy prove true, it will be the latest murder of someone linked to secret services since the killing of Alexander Litvinenko in 2006.
Litvinenko, a former KGB agent turned Kremlin critic, died in hospital after being poisoned with the radioactive substance polonium-210.
Britain has been known as a den of spy activity since the Cold War, with some 4,000 British intelligence specialists alone working for MI6, GCHQ or Britain's domestic security agency, MI5.
In 1978 Georgi Markov, a Bulgarian dissident, died of blood poisoning after he was stabbed with an umbrella at a London bus stop.