Beatles drummer Ringo Starr was the first to give the news, tweeting a picture of the "Fab Four" with Martin and saying: "God bless George Martin peace and love to Judy and his family... George will be missed".
Paul McCartney -- the only other surviving member of the band signed up by Martin in 1962 -- penned a tribute, calling him "a true gentleman" and "a second father".
"If anyone earned the title of the fifth Beatle it was George," McCartney wrote on his website.