But now the true identity of James Bond's mentor has finally been revealed - after a bit of sleuthing that even 007 himself would have been proud of!
An eagle-eyed fan watching the latest Bond movie Skyfall managed to freeze-frame a single camera shot lasting just a split-second...and discovered that M's real name was Olivia Mansfield.
Post-funeral, James Bond is handed a box containing a Royal Doulton bulldog draped in a British flag, which is usually kept on M's desk. If you look really REALLY carefully and quickly, zooming in on the box's inscription, you'll see the words 'From the Estate of Olivia Mansfield Bequeathed to James Bond'.
Since the revelation, internet forums have been filled with speculation about the significance of the name. One theory is that it's a tribute to Mansfield Smith-Cumming, first head of MI6 who started the tradition of the organisation's director signing letters with a single letter – in his case C. Others have speculated it is a portmanteau of 'man's field', alluding to the fact that M worked in a male-dominated industry.
Since Dame Judi Dench became Bond's first female spymaster in 1995, producers have dropped a series of hints about her character's name. You'll recall in Casino Royale (2006), 007 tells his boss: "I thought M was a randomly assigned letter. I had no idea it stood for *****." To which she interjects: "Utter one more syllable and I'll have you killed."
So now you know what your favourite secret agent has kept secret for the last seven years!