'Thank You, Sir Alex!'

The football world took to Twitter to react on Sir Alex Ferguson's retirement.

There was obviously shock and admiration after the legendary manager stepped down.

Manchester United broke the news on Twitter: Sir Alex Ferguson retires. #thankyousiralex'

- Ferguson retires as Manchester United manager

Newcastle United's defender Danny Simpson was one of the first to react.

"Wow I can't believe Sir Alex has actually retired. It doesn't seem real. He's an absolute legend n I'm privileged to say I worked with him."

Paul McGrath, who played under the 71-year-old Scot, tweeted: 'Sir alex ferguson . Respect .!'

Former Liverpool striker Stan Collymore wrote: 'Manchester United fans. Welcome to mortality again.'

Former Deputy Prime Pinister John Prescott tweeted: '#thankyousiralex' for helping the reds in football and politics. Legend.'

Former United striker Michael Owen tweeted, "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."

Rival Manchester City's Vincent Kompany wrote, "Sir Alex, one of the best managers of all time. After 26 years of success in the game, we all owe him a tribute."