Posted by Dave Thewlis on September 21, 2010 at 19:07:19 user dthewlis.
According to Keith Jeffery's just-published bio of MI6 1909-1949 (as recorded in today's Guardian), "Graham Greene, Arthur Ransome and Somerset Maugham all worked for MI6". Jeffery was actually given access to MI6's archives. So this may be actual confirmation of the oft-repeated claim, although the story in the Guardian doesn't offer any details.
http://www.guardian.co.uk/world/2010/sep/21/mi6-first-authorised-history.