Stephen Curtis, who died in 2004, was chairman of the security firm ISC Global (UK) which worked for a group of Russian tycoons plotting against Putin.

The dossier says the company was to “discredit [Putin] and those around him”. The targets were 11 senior Russians — from the defence minister to Roman Abramovich, owner of Chelsea football club.

ISC was also tasked with creating a luxury yacht with a crew capable of repelling an armed assault. The ship was to be a floating refuge for oligarchs wanted by Moscow on charges of fraud.

Curtis, 45, died in March 2004 alongside Max Radford, 34, the pilot, when their helicopter crashed near Bournemouth airport on the way to Dublin.

The wreckage yielded few clues and an inquest