Investigation into death of British spy Gareth Williams takes another mystifying turn

Detectives then called Mr Burk the following day, interviewing him for another hour by telephone. They gave him strict orders not to talk to the Press or the public. It is understood he was asked detailed questions about the former colleague of Mr Williams, who The Mail on Sunday has agreed not to name. The man is thought to have moved out of the flat six months ago.

The military

Set up to monitor the military and diplomatic communications of the Soviet Union during the Cold War, Echelon now eavesdrops on terror suspects and drug dealers, and searches for other political and diplomatic intelligence.

It is understood Mr Williams was part of a team of maths geniuses trying to adapt the 40-year-old Echelon system to deal with new forms of electronic communications.

According to sources, one of the big issues Mr Williams was working on was how the security and intelligence agencies can monitor internet telephone calls – known as Voice Over Internet Protocol (VOIP) – such as Skype, which are being used by terrorists and foreign agents to try to circumvent routine eavesdropping on telephone and mobile networks.

Two years ago Britain’s Intelligence and Security Committee, which oversees the work of Britain’s spies, revealed: ‘One of the greatest challenges for GCHQ is to maintain its intercept capability in the face of rapidly evolving communications technology.