Sir Alex Ferguson will retire at the end of this season, Manchester United have announced.
In a statement, Sir Alex said it was "the right time" to retire.
His last game in charge will be at West Bromwich Albion on May 19 and he will join the club's football board.
Real Madrid's Jose Mourinho and Everton's David Moyes are among the favourite to succeed him.
Sir Alex said: "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 organisation 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."
There was no hint of anything different as England's most successful football manager arrived at United's Carrington training complex just before 7am this morning amid massive speculation about his future.
But an announcement from the club at 9.15am confirmed he would be stepping down after guiding the Red Devils to a record 20th league title.
Last week, United privately confirmed Ferguson had been booked in for hip surgery in August, after the club's pre-season tour, which involves trips to Thailand, Australia, Japan and Hong Kong.
Repeatedly over the last few years, Ferguson has said his health would be the key to his own longevity.
On Monday night, there was a massive gamble on Moyes to join United in the summer.
Other names in the frame include Mourinho, Manchester United legend Ole Gunnar Solskjaer, Borussia Dortmund boss Jurgen Klopp and former France coach Laurent Blanc.
William Hill are giving odds of 50-1 on Hull City manager Steve Bruce taking over at Old Trafford.
Ferguson will leave as Britain's most successful manager in trophy terms by some considerable distance, having claimed his 49th piece of silverware with this season's title win.More to follow.
