Former Manchester United manager Alex Ferguson has undergone emergency surgery for a brain haemorrhage.
The English Premier League club said "the procedure has gone very well but he needs a period of intensive care to optimise his recovery. His family request privacy in this matter."
Ferguson, 76 retired from managing United in 2013 after 26 years in charge but he has continued to regularly attend matches and participate in UEFA coaching initiatives.
He is being treated at Manchester's Salford Royal Hospital.
His son Darren, manager of League One side Doncaster, did not attend his side's final home game of the season against Wigan due to his father's illness.
Ferguson remains the most successful manager in British football history, winning the Champions League twice, the Premier League 13 times and five FA Cups.
He was knighted in 1999, the year United achieved a treble by winning those three trophies in one season.
He made his managerial reputation in his native Scotland with Aberdeen, winning three Scottish League titles and the European Cup Winners' Cup.
After taking Scotland to the 1986 World Cup following the death of Jock Stein, Ferguson joined United in November 1986 after Ron Atkinson was sacked.
It took three and a half years to achieve his first success with them, the FA Cup title in 1990.
Ferguson was pictured on the Old Trafford pitch last Sunday making a presentation to former managerial adversary Arsene Wenger, who is preparing to leave Arsenal after almost 22 years.
Everton manager Sam Allardyce said: "It is tragic. I am going to find out how he is as soon as I possibly can. Obviously I knew it was extremely serious when I knew his son Darren didn't attend his own game at Doncaster today.
"I hope he is in good hands and I hope the operation is a major success because as a personal friend...I hope he has a full recovery."
Former Manchester United striker and Southampton manager Mark Hughes, who won two Premier League titles under Ferguson, said: "I heard some whispers and was hoping it wasn't true. I wish him all the best."
United midfielder Michael Carrick said on Twitter: "Absolutely devastated to hear about Sir Alex being unwell in hospital. All my thoughts and prayers are with him and his family. Be strong Boss."
Mike Phelan, who played under Ferguson for United and later served as his assistant manager, said on Twitter: "#SirAlex You've won more than most and if anyone can you can boss."