Mr Platini said: "Sir Alex has made a massive contribution to football, not only in Scotland and in England, but across Europe and beyond." The 71-year-old has led United for over a quarter of a century winning 13 English championships and two UEFA Champions League titles. "His dedication, his attention to detail and his unique eye for talent, as both the manager of Manchester United FC and Aberdeen FC, has brought rich rewards over a 30-year period. His CV is almost unique in a results-based profession that normally focuses on short-term solutions rather than long-term vision.
"He is a true visionary," continued the UEFA President, "and I hope that, having helped us in the past through various coaching initiatives, he will continue to collaborate with UEFA to share his fantastic knowledge with the next generation of up-and-coming European coaches who all wish to emulate his achievements in the sport."
UEFA.com rounds up the rest of the reaction from around Europe to Sir Alex's retirement.
FIFA President Sepp Blatter
His achievements in the game place him without doubt as one of the 'greats'. It was an honour to present Sir Alex with an award at the 2011 FIFA Ballon d'Or. Will his longevity at the top ever be repeated?
Marcello Lippi, former SSC Napoli, FC Internazionale Milano, Juventus and Italy coach
I didn't expect this decision, I have talked to him many times, even recently, and he kept repeating that he didn't want to go back to the 'chief', his wife. A person with such culture and intelligence can do any job he likes. It's fair to say he can become an ambassador for the entire world of football now, not only for United.
I don't speak English and they tell me that his English is peculiar but we started to communicate in French and we became good friends. We also started to exchange bottles, he used to send me whisky and I used to send him Italian wine. We talk about everything. I think his greatest achievement was to continue to change his teams and modernise them while continuing to win.
Peter Schmeichel, former Manchester United and Denmark goalkeeper
Disappointed. Shocked. Sad. I was not expecting today to be the day. Having known Sir Alex for more than 20 years, and from everything he has always said, it does not make any sense that he stops now.
Paul Ince, former Manchester United and England midfielder
I'm totally shocked. What he has done is unbelievable. You'll never see anyone of his kind again. Two weeks ago he was talking about staying on for another two years, so it's a massive, massive shock. The way he treated me was like a son.
Michael Owen, former Manchester United and England forward
It's just not sinking in. Manchester United with no Sir Alex just doesn't feel right. What a man, and great that it has ended on his terms. What a privilege to have played under arguably the best manager the world has ever seen. His record will surely never be eclipsed. Simply astonishing to read through his managerial record of achievements.
Michael Laudrup, former Juventus, FC Barcelona, Real Madrid CF and AFC Ajax midfielder, current Swansea City AFC manager
Unfortunately, the word legend is a word which is being used way too much these days, but in this case you can really talk about a legend in football, and therefore I am proud to have been Ferguson's last opponent [as Swansea boss] in his last home match as manager at Old Trafford.
Vincent Kompany, Manchester City captain
Sir Alex, one of the best managers of all time. After 26 years of success in the game, we all owe him a tribute.
Alex McLeish, former Aberdeen FC defender
He's just won Manchester United their 20th league title, so he probably saw it as the right time. I am incredibly proud to have worked with him, as he's been a huge part of my life both as a player and in management. I'm sure he'll still have a huge part to play in the future of the game. The next guy at Manchester United has massive shoes to fill.
David Bernstein, FA (Football Association) chairman
Sir Alex Ferguson's achievements are truly remarkable - he is genuinely one of the greatest managers of all time and certainly of the modern era. His contribution to English football has been outstanding in every regard and, in the FA's 150th year, it is something that should be celebrated. On behalf of the FA I'd like to wish him a happy and healthy retirement.
Neil Lennon, Celtic FC manager
It is a privilege to know Sir Alex and I would like to thank him for all the assistance he has given me as a manager. There is no better source of help and advice than Sir Alex Ferguson. He has decades of experience as a manager but most importantly he knows how to win.
He has given Manchester United fans a glorious period of success, bringing trophy after trophy to Old Trafford and making his club one of the biggest footballing institutions in the world. The resilience and commitment he has shown during this period as manager cannot be underestimated. While there were difficult times, his strength of character, passion and single mindedness meant he achieved such great things.
Ruud van Nistelrooy, former Manchester United and Netherlands forward
2001-2006, 219 matches, 150 goals under the most successful coach in the history of football. It was a unique privilege.
Edwin van der Sar, former Manchester United and Netherlands goalkeeper
It was coming for some time now but it's still a shock that hasn't sunk in yet. Great manager and remarkable person.
Bryan Robson, former Manchester United captain
For me you have to say he has been the greatest club manager ever in the world. He has rebuilt four or five squads to be successful, and look what he did at Aberdeen as well. Because of that record, he has to go down as the best.
Ottmar Hitzfeld, former Borussia Dortmund, FC Bayern München coach, current Switzerland coach
He has worked at Manchester United for so long, you don't even know who his predecessor was. It looks like he thought this would be the ideal moment to step down after winning the league. He will surely remain an ambassador for Manchester and the club.
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He may have been a top player, but he always was a twat