Former Manchester United and England captain Bryan Robson believes Sir Alex Ferguson is "probably the best club manager there has ever been".
.
Ferguson, 71, will step down as United manager at the end of the season after 26 years in charge in which he won the Premier League title 13 times.
.
"It is unbelievable to change around probably four different squads and have the success he has," Robson said.
.
Prime Minister David Cameron described Ferguson's record as "exceptional".
.
Mr Cameron, an Aston Villa supporter, added: "Hopefully his retirement will make life a little easier for my team."
.
Former Newcastle and England striker Alan Shearer, who almost signed for Manchester United from Blackburn in 1996, said: "If it wasn't for my love of Newcastle then I would've had to sign for Sir Alex. I was that close I'd actually found a house in Manchester.
.
"His knowhow, his desire, his hunger, his will to win and longevity are absolutely staggering.
.
"When you saw him last season squaring up to Mancini, at his age you have to admire that. He's an absolute genius. If you could bottle that it'd be worth a fortune."
.
Labour leader Ed Miliband tweeted: "Proud man.
.
Great manager. Staunch Labour Party supporter. Sir Alex Ferguson will never be forgotten."
.
Ferguson has won 38 trophies in total since taking over from Ron Atkinson on 6 November 1986, including this season's Premier League crown.
.
A host of former United players joined Robson, who was at United from 1981 until 1994, in paying tribute to Ferguson.
.
Sir Bobby Charlton spoke fondly of a "fantastic" and "sensational" manager.
.
"I am a director at United but I hardly do anything because we are winning all the time and it is all down to Sir Alex Ferguson," said Charlton, who made more than 600 appearances for United between 1956 and 1973.
.
"He would get up in the middle of the night and travel 300 miles if he thought there was a schoolboy he could sign. He loves the game."
.
Steve Bruce , who captained Ferguson's 1993-1994 double winning team, said he has a "wonderful humility", and that "he'd treat the groundsman the same as he would a star player."
.
Former England captain Paul Ince, who played under Ferguson for six years between 1989-1995, does not believe there will be another manager who will replicate Ferguson's achievements.
.
"You will never see anyone of his kind again," Ince said.
.
"His standards were so high. He was so demanding. Yes, we had our ups and down. The way he treated me was like a son."
.
Champions League winning goalkeeper Peter Schmeichel said he could not make sense of the timing of the announcement.
Of course so many Manchester United fans would not like this news at all. It came as a shock because no one saw it coming.
The retirement came as a shock to so many United fans. We wish it never happened. Let's pray for Moyes to continue from where Fergie stopped.