Former England manager Fabio Capello admitted he had been taken aback by Sir Alex Ferguson's announcement:
I met him a few months ago at the England Sports Centre and he told me he would carry on, so the news of his retirement surprised me.
I called him yesterday and he told me he has decided to go now because there is too much stress nowadays.
Always having to win something is very challenging, and he told me he wanted to go around the world with his wife.
David Moyes will hold his weekly press conference this afternoon but will not answer questions about Manchester United manager.
Loyal, driven and fiercely competitive - the traits of the new Manchester United manager David Moyes
David Moyes has emerged as the frontrunner to succeed Sir Alex Ferguson as Manchester United manager.