Neil Lennon has confirmed he has left Celtic after four years as manager and will help the club find his successor.
It is understood the former club captain had been considering his position for some time despite securing a third straight Scottish title.
The 42-year-old Northern Irishman told Celtic's website: "I have supported Celtic all my life and the club will always be part of me.
"However, I feel the time is now right to move on to a new challenge."
Lennon's statement came after meetings with his representative, Martin Reilly, and Celtic chief executive Peter Lawwell.
"It was an absolute honour to play for this great club, to captain the team and, of course, to become Celtic manager, a position which I cherished dearly," he said.
"Of course, I am very pleased that I will be leaving Celtic as Scottish champions and with the club in such good health.
"I wish the club nothing but good fortune going forward. I will now assist the club in appointing my successor."
Lennon took charge of the Glasgow club in 2010, leading them to three league titles and two Scottish Cups.
He also took Celtic to the Champions League last 16, with a victory over Barcelona in November 2012 his side's most high-profile achievement.
However, he had concerns about his playing budget for next season and is believed to have feared his present side would be unable to negotiate three rounds of qualifying.
Celtic chairman Ian Bankier had nothing but praise for the departing manager.
"Neil has given a huge part of his life to serving the club and we will always be grateful for all he has given us," he said.
"Ultimately, Neil brought Celtic back to the top of Scottish football, re-introducing the passion and drive which we needed to succeed and, of course, keeping us there for the last three years.
"Neil has shown the club a huge level of commitment and effort over his four years as manager."
With Lennon on a rolling contract, there had been annual speculation about his future and he is thought to have been close to leaving the club last summer.
West Bromwich Albion were believed to be aware of his availability before pursuing other targets, but are again seeking a new manager after the departure of Pepe Mel.
Lennon has often expressed his desire to eventually manage in the Premier League in England, but has been linked with the vacancy at relegated Norwich City.
Former Celtic defender Malky Mackay, sacked by Cardiff City last season, remains favourite to take charge at Carrow Road, with the Canaries having also interviewed fellow ex-Watford boss Gianfranco Zola and Neil Adams, who was given the job until the end of the season after Chris Hughton's exit.
Mackay will be among those linked with the vacancy at Celtic Park, although the early favourite with bookmakers is former Hoops striker Henrik Larsson, currently in charge of Swedish outfit Falkenbergs.
Speculation had heightened this summer about Lennon's position after his assistant, Johan Mjallby, confirmed he was leaving to "seek some different opportunities".
While Lennon's representative said that the Northern Irishman had "nothing lined up" at another club, Lawwell wished him future success.
"When we appointed Neil as manager, we did so knowing exactly what the club meant to him and knowing that he would give all he had to the job," said the chief executive.
"He has delivered great success to our supporters domestically and has brought us back to the table in European football.
"We now embark on a new and exciting chapter for the club."
A former midfielder who started his senior career with Glenavon in his homeland, Lennon played for Manchester City, Crewe Alexandra and Leicester City before joining Celtic in 2000.
He had a seven-year spell as a player at Celtic Park and returned as first-team coach in 2008 after short spells with Nottingham Forest and Wycombe Wanderers before taking over as manager in 2010 after time as caretaker boss.