An SAS sniper, lying on a bridge, points his long-range rifle towards a dual-carriageway and peers into his telescopic sight, as if poised to open fire. It makes for a startling image – all the more so since the picture was taken not in a conflict zone or even a training camp but in a public area in the Welsh countryside.

What makes it more arresting is that the photograph was found on a computer belonging to the Special Forces marksman known as Soldier N, who is said to have told his wife that members of the SAS ‘arranged’ the death of Princess Diana.

The Mail on Sunday can reveal that it has now been passed to the Metropolitan Police, whose specialist crime and operations command is investigating the sensational, if improbable, assassination theory.