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The book deflates some cherished myths. MI6 agents do not have a “license to kill,” although the agency compiled a list of possible Nazi assassination targets before the D-Day landings. It was judged that the plan was too risky and might spark bloody reprisals.

More happily for spy buffs, Q _ the gadget-making super-scientist from the Bond films _ is based on reality. After World War II, MI6 researchers worked on silent weapons, knockout tablets, safecracking tools and exploding filing cabinets that could destroy secret documents at short notice.

The book follows the publication last year of an official history of MI5, Britain’s domestic intelligence service.

Jeffery said he struck a “Faustian pact” when he agreed to write the book. He could look at everything in the archives, but MI6 retained the power to censor what was published.

The book stops abruptly in 1949, but still represents a change of policy for an agency whose existence was only officially acknowledged in the 1990s.

John Scarlett, the former MI6 chief who commissioned the book, said it is intended to “promote well-informed understanding and public debate about MI6,” without compromising current operations or living agents.

There is unlikely to be a sequel.

“For MI6 this is an exceptional event,” said Scarlett, who stepped down last year as “C,” code-name for the agency’s head. “There has been nothing like it before and there are no plans for anything similar in the future.”

The book is published in Britain by Bloomsbury as “MI6” and in the U.S. by Penguin as “The Secret History of MI6.”