Detectives searching for a missing British spy said Wednesday they launched a murder investigation after a body matching the man's description was discovered in his apartment near the headquarters of the MI6 spy agency.

British authorities who spoke on condition of anonymity because of the sensitivity of the investigation said the body of the man in his 30s was found stuffed in a bag in the bathroom Monday.

A florist delivers a pot of flower as a British Community Police Support officer stands quard outside a house in central London, near the headquarters of MI6, where a man believed to be a British spy was found dead in a flat. (Sang Tan/Associated Press)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. An autopsy and formal identification was expected later Wednesday.

The man had been working for MI6 on temporary assignment from GCHQ, Britain's eavesdropping agency, said several British officials who spoke on condition of anonymity.

They all declined to say what work the man was doing or how long he had worked for the government, saying he had yet to be formally identified and the investigation was ongoing.

Neighbour Rob Mills, a 35-year-old who lives two doors from where the body was found, said people in the expensive London neighbourhood of Pimlico knew little about the victim or his work.

"It's not like you'd tell your neighbours if you were a spy," he said.

Police cordoned off the area Wednesday and were restricting access to residents, some of whom said they were told by investigators that the man could have been killed two weeks ago. Scotland Yard refused to confirm.

"His windows were always shut and curtains were often closed," said neighbour Laura Houghton, 30. "I could never tell if anyone was in. It was strange that we never saw him come and go."

Houghton said the man was friendly and spoke with a Welsh accent.

Den of activity

Britain has been known as a den of spy activity since the Cold War, with 4,000 British intelligence specialists alone working for MI6, GCHQ or Britain's domestic security agency, MI5.

The Russians are thought to have hundreds of agents in London.

In 2006, the world was gripped by the story of the poisoning death of former Russian spy Alexander Litvinenko, who wasted away in a London hospital after ingesting a radioactive substance. On his deathbed, Litvinenko blamed Russian President Vladimir Putin for the poisoning.

His death hearkened back to the notorious 1978 killing of Bulgarian dissident Georgi Markov who died of blood poisoning after he was stabbed with an umbrella at a London bus stop. The tip of the umbrella was said to contain poison.

The largest spy swap since the Cold War made headlines last month when four people convicted of betraying Moscow for the West were pardoned in exchange for 10 Russian agents who had infiltrated suburban America. Two of them were flown to Britain.

Last month, a 21-year-old was arrested in connection with a parcel bomb being sent to MI6's fortress-like headquarters near the River Thames but authorities said Wednesday it appeared the that incident was unrelated to the discovery of the body.