Our thoughts are with his family, friends and all at Celtic FC at this very difficult time.
Billy joined Celtic in 1957 and became their greatest-ever captain during an incredible 18 years with the club, which saw him make almost 800 appearances for the Hoops.
He famously captained Celtic to European Cup glory against Inter Milan in Lisbon in 1967 and was also part of the team who were runners-up to Feyenoord three years later. His domestic trophy haul as a player was immense - nine league titles in a row under Jock Stein, seven Scottish Cups and six League Cups.
In a glittering playing career, he also earned 29 Scotland caps and was named Scottish Football Writers' Player of the Year in 1964/65.
Billy hung up his boots following the Scottish Cup win over Airdrieonians in 1975 and managed Clyde and Aberdeen before becoming Celtic boss in 1978.
He guided the team to five trophies and, after spells with Manchester City and Aston Villa, Billy returned for a second spell as manager and led Celtic to the double in their centenary year.