Neil Lennon resigned as Celtic manager on Wednesday after a disastrous season for the Glasgow giants, with fierce rivals Rangers on the brink of a first Scottish Premiership title in a decade.
The Northern Irishman 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.
Celtic were chasing a historic 10th consecutive top-flight title this season but their campaign was thrown into disarray on and off the pitch.
The reigning champions are 18 points behind Rangers and defeat to Ross County on Sunday left Steven Gerrard's men just seven points away from the title.
Celtic also crashed out of Europe in the Champions League qualifiers and Europa League group stages, leading to fan protests outside the club's stadium and online calls from supporters for Lennon to go.
"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 said in the statement: "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."
The Celtic hierarchy, led by chief executive Peter Lawwell, who is to stand 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 stepped in two years ago initially on a temporary basis when Rodgers left mid-season to take charge of Premier League side Leicester.
After inheriting an eight-point lead, the Northern Irishman 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 immediately offered Lennon the job permanently in the dressing room after the Scottish Cup final in a move that has since been fiercely criticised by fan groups.
However, Celtic completed another clean sweep in the 2019/20 season as Lennon's side built up a 13-point lead at the top of the table before the campaign was curtailed by the coronavirus pandemic.
A points-per-game formula was used to award a ninth straight title, matching the feats of Celtic's greatest-ever side from 1965 to 1974 and the big-spending Rangers team managed by Graeme Souness and Walter Smith from 1988 to 1997.
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.
However, 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 by a poolside enraged fans and when defender Christopher Jullien tested positive for coronavirus on Celtic's return to Glasgow, 13 first-team players plus Lennon and assistant manager John Kennedy were forced to self-isolate for 10 days.
A severely depleted squad lost further ground and Rangers' march to the title became a formality.
Kennedy will be put in caretaker charge with the task of denying the blue side of Glasgow the ultimate prize of winning the league when they visit Celtic Park on March 21.
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