Eddie Howe (left) ended his eight-year spell with Bournemouth in August
Eddie Howe has turned down the opportunity to be Celtic's new team boss, with the club blaming "reasons outwith both his and our control".
The deposed Scottish champions had hoped the 43-year-old would sign a contract in the next few days.
It is not clear why the ex-Bournemouth boss decided against the move, but it is understood the make-up of his backroom team could have played a part.
Celtic hope to appoint a successor to Neil Lennon "very shortly".
The Glasgow side finished the last three months of the season with the Northern Irishman's former assistant, John Kennedy, in interim charge.
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"Following very positive and detailed discussions with Eddie Howe, with the belief that he would be an excellent candidate for the position of Celtic manager, we allowed time for the process, given he'd previously made it clear he was not looking to return to management until this summer at the earliest," the club said in a statement.
"We can now confirm that Eddie will not be joining the club. We fully appreciate our fans will be seeking clarity on this matter."
Celtic said they had "engaged with a number of candidates" and hope to soon "announce the appointment of a manager with the quality and desire to take Celtic forward".
As they gave an update to supporters at the launch of season ticket sales for next season, Celtic on Wednesday apologised for the delay in making an appointment and said they intended to "finalise" the appointment of their next manager shortly.
However, Celtic, who had followed four consecutive domestic trebles by ending a campaign without a trophy for the first time since 2010, are looking elsewhere after talks that have been ongoing with Howe since April finally broke down.
The Englishman had ended his eight-year spell as Bournemouth manager, his second at the club, in August by mutual consent after their relegation from the Premier League.
His former Bournemouth team-mate, Peter Grant, refused to discuss if he was offered a place in any new management team at Celtic Park when he was being confirmed as Dunfermline Athletic's new manager shortly before news broke about talks breaking down with Howe.
Meanwhile, Ange Postecoglou, the 55-year-old Greek former Australia head coach who has been with Yokohama F Marinos in Japan since 2018, has emerged as the new favourite for the Celtic job with some bookmakers.
'It's a bad look for Celtic' – analysis
Former Celtic striker Chris Sutton on BBC Radio 5 Live
I find it odd, the backroom staff story, because surely, if they've been in talks for a long, long time – this seems to have been ongoing for weeks if not months, Eddie Howe would have nailed down his backroom staff and that would have been a done deal.
We've seen incidents in the last couple of weeks in Glasgow, the chief executive of Celtic, Peter Lawwell, his house had a petrol bomb and his car was set alight, there was trouble in the centre of Glasgow at George Square.
I just wonder, he has a wife, he has a family and I wonder if they think of things like that and think, I don't want to go up to Glasgow and be involved in that.
What we do know is that this is a bad look for Celtic as they haven't got this over the line. We are 50 days before a Champions League qualifier and it is a big blow.