Celtic manager Neil Lennon has resigned with his side 18 points adrift of Rangers in the Scottish Premiership.
Lennon took charge for a second spell in February 2019 when Brendan Rodgers left and led the club to two titles, two Scottish Cups and a League Cup.
However, Celtic are well behind their Glasgow rivals in their bid to win 10-straight titles and slumped to defeat at struggling Ross County on Sunday.
Assistant manager John Kennedy will take interim charge of the team.
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"We have experienced a difficult season due to so many factors and it is very frustrating and disappointing that we have not been able to hit the same heights as we did previously," Lennon said.
"I have worked as hard as ever to try and turn things around, but unfortunately we have not managed to get the kind of run going that we have needed.
"I have always given my best to the club and have been proud to deliver silverware to the Celtic supporters. The club will always be part of me. I will always be a Celtic supporter myself and I will always want the best for Celtic."
'It was inevitable' - reaction
Celtic chief executive Peter Lawwell: "Neil has always been and will always be a true Celtic man and someone I will always hold in the highest regard. Personally, it is a sad day for me to see Neil leave. Neil is a man of quality and decency, he is someone who will always be part of the fabric of Celtic and someone who will always be welcomed."
Principal shareholder Dermot Desmond: "Neil is a Celtic legend both as a player and manager. He has given so much to the club, and with his success over the last number of years - including winning a treble in his own right - he will be very difficult to replace."
Celtic captain Scott Brown: "He's supported Celtic his whole life and he wanted to play for this club - he won a treble as a player, as well as a manager. He's given 15-20 years to this club and he deserves respect, and also praise for what he's done for the club and the success he brought us. I probably wouldn't still be captain at Celtic if it wasn't for Neil Lennon."
Former Celtic defender Gary Caldwell: "It was inevitable. The results just haven't been good enough and I feel for Neil because I was there as a player with him and he's given his life to Celtic. But the club had to make a decision to move forward."
Rangers manager Steven Gerrard: "I can't say I'm surprised because of what's been going on over there. But I obviously take no personal satisfaction, so I wish him well for the future."
Former Celtic manager Brendan Rodgers: "When it all settles down, Neil is an incredible legend of the club, he has given so much service to the club, won so many titles and like I say, his legacy there will be intact forever."
Former Rangers manager Ally McCoist: "Hopefully he can get some sanity back in his life and go and enjoy himself and relax with his family for a period. The season for Celtic has been one disaster after another, and there are a lot of people responsible for that."
Ross County manager John Hughes: "My heart goes out to Lenny. I pay tribute to his accomplishments and success he has had. He will be hurting."
'Surprise it took this long' - analysis
BBC Scotland's sports news correspondent Chris McLaughlin
This is something we've been expected for some time, and it comes as a surprise that it hasn't happened before now, even for the man himself. The defeat at Ross County seems to have been the last straw. The Celtic board have been in talks about this over the past few days.
Lennon delivered the unprecedented quadruple treble, but he has had major struggles with the side this season and Rangers are now just seven points away from stopping Celtic making it 10 titles in a row.
The majority of the fans have been calling for him to step aside for some time now.
It will take time for the dust to settle but I would imagine in the next couple of seasons, Celtic fans will look back with fondness on Neil Lennon as both player and manager.
How did it get to this stage?
Lennon answered Celtic's call in early 2019, when Rodgers departed mid-season for Leicester City, and guided the club to their eighth league crown in a row while securing a third-straight Scottish Cup.
After Celtic beat Hearts in the final, he was offered the job on a permanent basis by Lawwell in the Hampden showers.
Last season was called early amid the coronavirus pandemic, with Celtic handsomely ahead of Rangers and crowned champions, having also defeated their rivals in the League Cup showpiece.
But this term, as the club pursued an unprecedented 10th league title in succession, things unravelled spectacularly.
Celtic were eliminated from the second round of Champions League qualifiers at home by Ferencvaros, and won just one of their six Europa League group games.
Although they won nine of their opening 10 league fixtures, they lost the first two Old Firm derbies of the term, and dropped points to Aberdeen, Hibernian, St Johnstone and Livingston.
Their incredible run of 35 domestic cup wins was ended by Ross County, who prevailed 2-0 at Celtic Park in the League Cup last 16. That led to fan protests and clashes with police outside the stadium.
Lennon oversaw more Scottish Cup success in December, as Celtic won the 2019-20 trophy on penalties despite losing a 2-0 lead against Hearts, ensuring the Northern Irishman became the first man to win a treble as player and manager, and sealed a Scottish-first quadruple treble.
But the glory was short-lived. Celtic lost at home to St Mirren in January, and despite holding games in hand, remained well shy of Rangers, who are yet to lose a league match this season.
Sunday's 1-0 loss to County, who were bottom of the table heading into the fixture, sealed Lennon's fate, with Rangers on the brink of ending their domestic supremacy.