Here is his farewell statement in full:
"The decision to retire is one that I have thought a great deal about and one that I have not taken lightly. It is the right time.
"It was important to me to leave an organization in the strongest possible shape and I believe I have done so. The quality of this league winning squad, and the balance of ages within it, bodes well for continued success at the highest level whilst the structure of the youth set-up will ensure that the long-term future of the club remains a bright one.
"Our training facilities are amongst the finest in global sport and our home Old Trafford is rightfully regarded as one of the leading venues in the world.
"Going forward, I am delighted to take on the roles of both Director and Ambassador for the club. With these activities, along with my many other interests, I am looking forward to the future.
"I must pay tribute to my family, their love and support has been essential. My wife Cathy has been the key figure throughout my career, providing a bedrock of both stability and encouragement. Words are not enough to express what this has meant to me.
"As for my players and staff, past and present, I would like to thank them all for a staggering level of professional conduct and dedication that has helped to deliver so many memorable triumphs. Without their contribution the history of this great club would not be as rich.
"In my early years, the backing of the board, and Sir Bobby Charlton in particular, gave me the confidence and time to build a football club, rather than just a football team.
"Over the past decade, the Glazer family have provided me with the platform to manage Manchester United to the best of my ability and I have been extremely fortunate to have worked with a talented and trustworthy Chief Executive in David Gill. I am truly grateful to all of them.
"To the fans, thank you. The support you have provided over the years has been truly humbling. It has been an honour and an enormous privilege to have had the opportunity to lead your club and I have treasured my time as manager of Manchester United."
"All the Best Sir Alex Ferguson"
Sir Alexander Chapman "Alex" Ferguson, (born 31 December 1941) is a Scottish football manager and former player who has managed Manchester United since 1986. His time at the club has led to Ferguson being regarded as one of the most admired and respected managers in the history of the game.
Ferguson managed East Stirlingshire and St. Mirren before a highly successful period as manager of Aberdeen. After briefly managing Scotland following the death of Jock Stein, he was appointed manager of Manchester United in November 1986.
Ferguson is the longest serving manager of Manchester United, overtaking Sir Matt Busby's record on 19 December 2010, and the longest serving of all current League managers. He has won many awards and holds many records including winning Manager of the Year most times in British football history. In 2008, he became the third British manager to win the European Cup on more than one occasion. He was knighted in 1999 for his services to the game and also holds the Freedom of the City of Aberdeen.
On 8 May 2013, Ferguson announced his retirement as manager of Manchester United active on 1st July 2013. During his 26 years at the club, he won 38 trophies, including 13 Premier League and two UEFA Champions League titles.