MI6 spy Gareth Williams may have died at the hands of a mystery bondage sex partner he met on London’s gay scene, detectives have suggested.

Investigators said the 31-year-old codebreaker was immersed in the capital’s thriving gay nightlife and had repeatedly visited extreme bondage websites. He also had a secret passion for fashion and hoarded unworn women’s designer clothes worth £15,000 in his wardrobe alongside several wigs.

Police said it would have been impossible for the dead man to lock himself in the holdall where his naked body was found. And they revealed forensic evidence indicating other people were in his Pimlico flat who they have been unable to trace.

But police said it is impossible to say whether Mr Williams was already dead when he was put in the bag or died from suffocation once zipped inside. Detective Chief Superintendent Hamish Campbell said the spy’s secretive private life probably holds the key to his death. He added that a “small sub-group of the community” may well know something about his final moments but police cannot trace them.

Speaking at New Scotland Yard, Mr Campbell said: “We are very sure that someone else was in that flat. We want to know the circumstances when you would leave somebody in that position, by accident or design. Maybe, by explaining to the public, someone will think: ‘I get it and I can explain’.”

Mr Williams, of Anglesey, North Wales, was found by police at his top-floor flat in Alderney Street, Pimlico, on August 23. Police believe he died in the early hours of August 16 and he was last seen the previous day returning from a shopping trip to Harrods. His decomposing body was in a large North Face holdall sealed by a travel-style Yale padlock through the zip fasteners. The keys were inside, under his body. The mathematics prodigy worked as a cipher and codes expert for GCHQ, the government listening station, but had been on secondment to MI6. The death sparked a frenzy of speculation as a battery of post-mortem tests and a huge police inquiry failed to determine how he died.