Neil Lennon has resigned as Celtic manager, the Scottish Premiership club have announced.
Lennon, who returned to the club for a second stint in charge after Brendan Rodgers left to take over at Leicester in February 2019, has been under pressure all season but had previously maintained he would not quit.
The reigning champions are 18 points behind Rangers in the top flight and a defeat to Ross County on Sunday left their Glasgow rivals just seven points away from title.
Lennon apologised for "letting fans down again" after the defeat by Ross County left Celtic's bid to win a 10th consecutive title in disarray.
The loss also heightened the prospect of Rangers clinching the title at Parkhead on 21 March.
Since returning to the club, Lennon has led Celtic to two titles, two Scottish Cups and a League Cup.
A club statement said: "Celtic Football Club today announced that Neil Lennon has resigned from his position as football manager with immediate effect.
"Neil has served the club with distinction as both player and manager, delivering numerous successes, most recently completing the domestic treble in December.
"In his second period as Celtic manager, he has achieved five trophy successes, to add to his three League titles and two Scottish Cup victories in his first period as manager."
Celtic chief executive Peter Lawwell said: "I would like to pay tribute to Neil for all he has done for the club in his second spell, delivering our eighth and ninth successive league titles, the quadruple treble and winning the last five available domestic trophies.
"Neil has always been and will always be a true Celtic man and someone I will always hold in the highest regard.
"I have watched Neil fight many battles over many years, on and off the field, with a courage and tenacity few could match. Even this season, he has fought so hard and worked tirelessly to turn things around.
"While this season has not progressed as we would have liked, it cannot diminish the character or integrity of a man who has given the club so much.
"Personally, it is a sad day for me to see Neil leave the club. 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 at Celtic Park.
"On behalf of everyone at the club, and personally, I would like to thank Neil for his work as our manager and I wish him and his family good health and continued success in everything they do."