Strachan in for O'Neill at Celtic
Celtic manager Martin O'Neill is to leave the club after Saturday's Scottish Cup final, with Gordon Strachan taking over from 1 June.
"I am leaving Celtic purely for personal reasons and I am extremely sorry to be departing in such circumstances," said O'Neill.
O'Neill's wife Geraldine has been receiving treatment for cancer.
Strachan has signed a 12-month rolling contract - the same arrangement as O'Neill had.
O'Neill's assistants John Robertson and Steve Walford are also leaving.
"We (the O'Neill family) have had discussions for some weeks as we awaited certain biopsy results," he added.
"We have had suspicions in recent weeks and that has taken it to a different level but she wouldn't like me to go into too much detail.
"We had some good news at the back end of last year and she is not too clever at the minute. I don't have a monopoly of bad times but it is the correct thing to do.
"She has stood with me for quite some time so this is the right thing.
"I would like to thank sincerely Dermot Desmond and the Celtic board for giving me the opportunity to manage this fantastic club five years ago.
"It has been an honour and privilege to have served the club and its supporters during that time and being part of Celtic history.
"My time at Celtic has been most rewarding and I have been privileged to have worked alongside a large number of very talented and committed people throughout the club.
"My special thanks to John Robertson and Steve Walford."
Chairman Brian Quinn said it is with "great regret" and "great reluctance" that O'Neill is going.
He added: "Whilst it will come as a great disappointment that he is leaving, we fully respect the reasons behind his decision.
"He had proved himself one of the great Celtic managers and his place in the history of the club is assured."
Chief executive Peter Lawwell said: "It's to the great credit of Martin and his assistants Steve and John that they have remained totally focused on their roles. They are consummate professionals.
"Martin has taken Celtic on a remarkably thrilling and successful adventure over the past five years and, as well as achieving tremendous domestic success has helped put Celtic back where it belongs - on the European stage."
"I hope everything goes well for him and his family but, at the end of the day, it is his decision and he knows what is best."
O'Neill has been in charge at Celtic since June 2000.
However, his side relinquished their hold on the SPL title on Sunday as they lost 2-1 to Motherwell and Rangers beat Hibernian.
Strachan spent nearly five years in charge of Coventry, from 1996 to 2001, having initially been player-coach under Ron Atkinson.
But his spell there ended unhappily when the Sky Blues were relegated from the Premiership - and he left the club.
However, the Scot was not out of football for long, as he was appointed Southampton manager in October 2001 following the unsuccessful seven-month reign of Stuart Gray.
Strachan spent just under two-and-a-half years with the Saints - leading them to the FA Cup final in 2003 - before announcing he was taking a break from football to have a hip operation.
From BBC Sport