The news was confirmed in a statement released this afternoon by Brian's family, including his brother and former Reds team-mate Jimmy. The siblings famously won the FA Cup together in 1977, as members of the United team managed by Tommy Docherty.
The club responded: "Everyone at Manchester United is shocked and deeply saddened by the news. Our thoughts and prayers are with Brian's family at this difficult time."
Brian made his debut for the Reds in September 1973 and went on to register 271 appearances, scoring 17 goals. He earned 18 senior caps for England and also played club football for Leeds United and Rochdale after leaving Old Trafford in 1979.