Neil Lennon resigned as Celtic manager on Wednesday with the Scottish Premiership club 18 points behind bitter rivals Rangers in their bid to win a historic 10th consecutive league title.
The Northern Irishman took charge for a second spell in February 2019 and led the club to two league titles, two Scottish Cups and a League Cup.
But he has been under pressure all season as problems mounted on and off the pitch, with Hoops' fans calling for his head as their dream died.
Celtic flopped badly in Europe and defeat to lowly Ross County on Sunday left Steven Gerrard's Rangers just seven points away from landing the club's first title for a decade.
"Celtic Football Club today announced that Neil Lennon has resigned from his position as football manager with immediate effect," said a club statement.
"Neil has served the club with distinction as both player and manager, delivering numerous successes, most recently completing the domestic treble in December."
Lennon, 49, said in the statement he had worked hard but had been unable to turn the club's fortunes around.
"We have experienced a difficult season due to so many factors and, of course, it is very frustrating and disappointing that we have not been able to hit the same heights as we did previously," he added.
Assistant manager John Kennedy will take interim charge of the team until a permanent successor is appointed.
The Celtic hierarchy, led by chief executive Peter Lawwell, who is standing down at the end of the season, stood by their manager long after realistic hope of catching Rangers was gone.
But Sunday's 1-0 defeat to Ross County, who started the game bottom of the table, proved the final straw.
- Trophy haul -
Lennon, who won 10 league titles with Celtic as a player and manager, stepped in two years ago initially on a temporary basis when fellow Northern Irishman Brendan Rodgers left to take charge of Premier League side Leicester.
He saw Celtic over the line in the league as Rangers faltered in Gerrard's first season in charge at Ibrox and won the Scottish Cup to complete a third consecutive treble of domestic trophies.
Lawwell offered Lennon the job permanently in the dressing room after the Scottish Cup final in a move that was fiercely criticised by fan groups.
Celtic completed another clean sweep in the 2019/20 season, with Lennon's side crowned champions under a points formula after the campaign was curtailed due to the coronavirus pandemic.
It meant they had matched the feats of Celtic's greatest-ever side, who won nine consecutive titles starting in the mid-60s, and the big-spending Rangers team that dominated from the late 1980s.
In December, victory on penalties over Hearts completed a fourth consecutive treble in the delayed 2019/20 Scottish Cup final and Lennon became the first man to win the Scottish domestic treble as a player and manager.
But by then the wheels had already come off Celtic's season with a string of disappointing results in Europe and Rangers building a commanding lead in the league.
A 1-0 victory for Gerrard's men in the second Old Firm derby of the season on January 2 opened up a 19-point lead.
Worse was to come for Celtic as Lennon's squad immediately jetted out from defeat at Ibrox for an ill-judged mid-season training camp in Dubai.
Pictures of Lennon and captain Scott Brown drinking beer poolside enraged supporters and when defender Christopher Jullien tested positive for coronavirus on Celtic's return to Glasgow, 13 first-team players plus Lennon and Kennedy were forced to self-isolate.
A severely depleted squad lost further ground and Rangers' march to the title became a formality.
Gerrard wished Lennon well, saying he took "no personal satisfaction" from his struggles.
Former Celtic striker John Hartson said Lennon could be proud of his achievements at the club.
"I'm not surprised he has resigned but feel it's the best outcome for Celtic," he tweeted. "The club now need to move quickly and get a new manager and his staff in for the challenges ahead."