The death of MI6 spy Gareth Williams was not the result of assassination by the security services, police investigating the incident believe.

In one of the most bizarre cases of recent times, Mr Williams originally from Anglesey, North Wales, was found naked and locked inside a sports bag inside his home in central London in August 2010.

A source close to the investigation has told the Telegraph, police believe it is most likely that Mr Williams died alone during “auto-erotic activity”.

Earlier this year, Coroner Dr Fiona Wilcox dismissed the suggestion and said his death was found to be “unnatural and likely to have been criminally mediated.”

An inquest heard that Mr Williams, a codebreaker for GCHQ who was on secondment to MI6, had previously been found in his boxer shorts and tied to his bed by his landlord and landlady in Cheltenham a few years earlier.

He kept pictures of drag queens on his computer and had £20,000-worth of designer women’s clothes in his flat along with women’s shoes and wigs.

His iPhone, found in his work locker, contained deleted images of him naked in boots posing with his back to the camera.

Friends and family were upset at speculation that Mr Williams may have been gay and suggested “some agency specialising in the dark arts” was behind his killing.

In her ruling, Dr Wilcox said there was no evidence to suggest the spy was a transvestite “or interested in any such thing”.

The make-up found in his flat was more likely to reflect his interest in fashion and the wigs were “far more consistent with dress-up such as attendance at a manga conference”, she said.

The suggestion that his interest in female footwear could have been of a sexual nature, was not unusual, Dr Wilcox observed.

On Thursday, a Metropolian Police spokesman said: “This remains an active investigation and officers continue to explore a number of lines of enquiry. Officers retain an open mind in relation to the circumstances surrounding the death of Mr Williams.”

Detectives have struggled to work out how the body of the MI6 codebreaker ended locked up inside the small bag along with the keys.