MI6 lifted somewhat the veil of secrecy which has surrounded its operations for the past century with the publication today of the first authorised history of the service.
Professor Keith Jeffery, of Queen's University, Belfast, was given unrestricted access to the surviving historic files of the Secret Intelligence Service (SIS), as it is more properly known.
At a launch at the Foreign Office today, Sir John Scarlett, the former SIS chief who commissioned the book to mark its centenary last year, said it was a "radical step" for an agency whose watchword is secrecy.
"Mansfield Cumming (the first service's first chief) believed passionately in secrecy. I am sure he would be surprised to see me here today presenting a history of his service," Sir John said.
"For MI6, this is an exceptional event. There has been nothing like this before and there are no plans for anything similar in the future.
"Although for much of its history it was astonishingly underfunded and very much smaller than imagination would have it, the overall impression one is left with is the remarkable level of achievement against a very wide range of extremely difficult and stressful intelligence targets on five continents."
Unlike the recent authorised history of MI5, which runs to the present day, it only covers the first 40 years of the service from 1909 to 1949.
Prof Jeffery also had to agree to a number of restrictions on what he could write - including a proviso that he could not name or allude to any agent whose identity was not already clearly in the public domain.
While he said that his "Faustian pact" had in some cases "overridden the imperatives of historical scholarship", it had not "materially undermined" his ability to tell many important stories from the period.
The include the exploits of such legendary characters as Sidney Reilly, the self-styled "Ace of Spies", and Wilfred "Biffy" Dunderdale, who knew author Ian Fleming and is one possible model for James Bond.
Prof Jeffery said he was able to lay to rest the myth that MI6 had a "licence to kill", although "fatalities" did occur in the course of its work, particularly during wartime.
"I looked very hard for 'bad stuff'," he said. "In the end, I found less evidence than perhaps we might have expected, certainly less evidence than I might have expected as the amateur espionage fiction buff that I was."
He said he found no evidence to support recent claims that MI6 was involved in the assassination in 1916 of Rasputin, the notorious "mad monk" who had insinuated himself into the Russian royal family.
"If MI6 had a part in the killing of Rasputin, I would have expected to have found some trace of that," he said.
The book does however refer to a colourful account of the murder by MI6's man in Moscow, Sir Samuel Hoare - a future government minister - who said he was "writing in the style of the Daily Mail" because it was "so sensational that one cannot describe it one would if it were an ordinary episode of the war".
"True to his nickname ('the rake') it was at an orgy that Rasputin met his death," Sir Samuel wrote.
The book also dismisses the story that Mansfield Cumming used a penknife to hack off his own leg after he became trapped following a car crash in which his son died. In fact, Cumming recorded that it was amputated.
Prof Jeffery acknowledged that one issue he was not able to cover was the treachery of Kim Philby, one of the Cambridge spies who served with MI6 in the 1940s, but who did not come under suspicion as a Russian agent until after the period covered by the book.
"He was the one area which I wrote about with the benefit of hindsight," he said. "With Philby, it goes kind of well and then there is a dot, dot, dot."