MI6 boss Sir John Scarlett still signs letters in green ink

Sir John said that he keeps a special coloured pen in his desk for official correspondence and to inspire junior agents with the history of the service.

MI6 bosses have been known colloquially as the "green ink brigade" since the days of Sir George Mansfield Smith-Cumming, the former Royal Navy officer who established the Secret Intelligence Service (SIS), also known as MI6, to gather intelligence on the Germans before the First World War.

A man who revelled in the secrecy and glamour of espionage, Sir George signed all his correspondence with his final initial in green pen – the writing implement of choice for conspiracy theorists and cranks.

His successors in the role adopted the convention and in a rare interview the current incumbent Sir John revealed that it continues to this day.

"That's a tradition that we have kept," he told BBC Radio 4's MI6: A Century in the Shadows.

"If you get a note of congratulation – a personal note from the chief – and instead of being just in ordinary ink signed John Scarlett, it is in green ink and is signed C it just makes a better impact. "

Sir John spent his career in MI6 before leaving in 2001 to head the Government's Joint Intelligence Committee.

He returned to the agency as director-general in 2004, but is due to step down in November and be replaced by Sir John Sawers, Britain's current ambassador to the United Nations.