Sir Alex Ferguson is to retire at the end of the season.
United confirmed the news on their website this morning.
"The decision to retire is one that I have thought a great deal about and one that I have not taken lightly," said Ferguson.
"It is the right time."
Ferguson's last game in charge will be at West Brom on May 19 and he will join the club's football board.
"It was important to me to leave an organisation in the strongest possible shape and I believe I have done so," he said.
"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."
Ferguson has had a career the like of which will never be seen again. He won his 49th trophy in United's 20th league championship and seemed set to extend his career into next season, confirming as much in his programme notes ahead of Sunday's encounter with Chelsea.
Instead, with a hip operation booked in for August, this weekend's Old Trafford encounter with Swansea, that will be followed by a trophy presentation, will be his last home game in charge.
He leaves behind a club in good shape.
"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," said Ferguson.
"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."
Although there have been times - in 1989 when it seemed he was going to be sacked, and 2002, when he was about to retire and instead performed a last-minute U-turn - when it was felt Ferguson was ready to leave, this time it is permanent.
The speculation intensified on Tuesday afternoon and United's failure to respond to questioning about the matter further fuelled the rumours overnight.
Now, Ferguson is left to look back on the power-shift in English football that he has instigated.
"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," said Ferguson.
"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."
Now, for the first time in over a quarter of a century, United are on the hunt for a new manager.
David Moyes, out of contract at Everton, and Jose Mourinho, who is expected to leave Real Madrid at the end of the season, are the obvious favourites, although Borussia Dortmund's highly-regarded coach Jürgen Klopp and Bayern Munich's 67-year-old outgoing boss Jupp Heynckes will also come into contention.
It promises to be a major test of nerve for the Glazer family, plus Ed Woodward, who will replace David Gill as chief executive in the summer.
"Alex has proven time and time again what a fantastic manager he is but he's also a wonderful person," said co-chairman Joel Glazer.
"His determination to succeed and dedication to the club have been truly remarkable.
"I will always cherish the wonderful memories he has given us, like that magical night in Moscow."
Avi Glazer added: "I am delighted to announce that Alex has agreed to stay with the Club as a director.
"His contributions to Manchester United over the last 26 years have been extraordinary and, like all United fans, I want him to be a part of its future."
Sir Alex Ferguson factfile:
1941: Born December 31 in Govan, Glasgow.
1957: Joins Queen's Park as amateur while apprentice toolmaker in Glasgow factory.
1960: Joins St Johnstone as part-timer.
1964: Quits toolmaking to join Dunfermline.
1967: Moves to Rangers for £65,000.
1969: Another move, this time to Falkirk, for £20,000.
1973: Joins Ayr, returning to part-time ranks while running his Glasgow pub.
1974: Appointed manager of East Stirling in September, but moves to St Mirren three months later.
1978: Sacked by St Mirren and appointed at Aberdeen as successor to Billy McNeill.
1980: First managerial honour as Aberdeen win Scottish championship.
1982: Aberdeen win the Scottish Cup, beating Rangers 4-1.
1983: Aberdeen retain Scottish Cup, this time beating Rangers 1-0, and defeat Real Madrid 2-1 in Gothenburg to lift European Cup Winners' Cup.
1984: Aberdeen win League and Cup double, beating Celtic 2-1 in the Cup final. Ferguson awarded OBE.
1985: Appointed caretaker manager of Scotland following death of Jock Stein during Wales v Scotland World Cup qualifier.
1986: Scotland bow out of Mexico World Cup after first round. Ferguson leaves Aberdeen to take over at Manchester United after sacking of Ron Atkinson.
1989: Breaks British transfer record to sign Gary Pallister for £2.3million from Middlesbrough.
1990: First trophy at Old Trafford as United beat Crystal Palace 1-0 in an FA Cup final replay after a 3-3 draw.
1991: United beat Barcelona 2-1 in Rotterdam to win Cup Winners' Cup.
1992: European Super Cup arrives at Old Trafford as United beat Red Star Belgrade, while first League Cup triumph booked with 1-0 defeat of Nottingham Forest. Championship dream dies as Leeds overhaul United in final weeks of the season.
1993: Old Trafford's 26-year wait for title is ended as United finish 10 points clear of Aston Villa to win inaugural Premier League title; Signs Roy Keane from Nottingham Forest for British transfer record £3.75million.
1994: United become only sixth team to complete championship and FA Cup double.
1995: Breaks British transfer record again to sign Andy Cole from Newcastle for £7million; United finish runners-up to Blackburn in the league and Everton in the FA Cup.
1996: United become the first club ever to complete the championship and FA Cup double twice, overcoming one-time runaway leaders Newcastle in the Premier League and then beating Liverpool 1-0 at Wembley.
1997: Claims fourth championship title in five seasons.
1998: Finishes season trophyless as Arsenal win double.
1999: Leads United to the treble of European Cup, Premier League and FA Cup; Knighted in the Queen's Birthday Honours.
2000: Guides United to a sixth Premier League title, finishing the campaign 18 points clear of Arsenal.
2001: Wins seventh title in nine years.
2002: Changes plans to retire to sign a new three-year deal as United manager; United finish outside top two in Premier League for first time.
2003: Involved in dressing room bust-up with star player David Beckham, who it emerges was cut in the face by a boot kicked by Ferguson in frustration; Courts controversy by claiming Champions League draw is fixed; United win title again; Launches a legal action against major United shareholder John Magnier over stud rights to top racehorse Rock of Gibraltar.
2004: United win FA Cup with victory over Millwall in final; Signs Everton star Wayne Rooney for a fee which could rise to £27million.
2005: United lose FA Cup final to Arsenal on penalties.
2006: Wins the League Cup for only the second time thanks to a 4-0 thrashing of Wigan.
2007: Wins first Premier League title for four years but United are denied the double after FA Cup final defeat to Chelsea.
2008: Beats Chelsea to the Premier League title again before defeating the Blues on penalties in the Champions League final in Moscow. United subsequently add the Club World Cup before the end of the year.
2009: Retains Premier League title having already won the League Cup against Tottenham. United reach Champions League final in Rome but lose to Barcelona 2-0.
2010: Forced to settle for just the League Cup, against Aston Villa, as Carlo Ancelotti helps Chelsea reclaim the title; becomes United's longest-serving manager.
2011: Given a five-match touchline ban and a £30,000 fine for his verbal attack on referee Martin Atkinson at Chelsea in February. "You want a fair referee, or a strong referee anyway - and we didn't get that," said Ferguson in the aftermath of the 2-1 defeat. "I must say, when I saw who the referee was I feared it. I feared the worst."
2012: May - United are pipped to the Premier League title on a dramatic final day of the season, with bitter rivals Manchester City instead taking top spot on goal difference.
September - Ferguson manages his 1000th league game with United against Southampton. Two weeks later, he wins his 100th game in the Champions League be beating Galatasaray at Old Trafford.
2013: April 22 - United land a record 20th league title with a 3-0 win over Aston Villa.
May 8 - United announce he will retire at the end of the season.