Which letter is the codename for the MI6 leader in James Bond stories?

M is a codename held by a number of fictional characters in Ian Fleming's James Bond book and film series; the characters are the current or past heads of the Secret Intelligence Service—also known as MI6. Fleming based the character on a number of people he knew who commanded sections of British intelligence. M has appeared in the novels by Fleming and seven continuation authors, as well as appearing in twenty-four films.

Source: Wikipedia