Sam Allardyce has ended his eight-year reign as manager of Bolton after resigning with immediate effect.
He has been linked with the manager's role at Manchester City and did not attend the post-match press conference after Saturday's 2-2 draw with Chelsea.
"This has been one of the hardest decisions I've ever made but it's time for a new exciting era for Bolton under different leadership," he said.
Chairman Phil Gartside added: "His reasons are private, we respect that."
Gartside later revealed he had known about Allardyce's intentions for the past fortnight, when the subject came up as part of planning discussions for next season.
"It became clear that he wanted some time with his family and to reflect on his life," Gartside explained.
"It's a complicated decision. Bolton has been his spiritual home for 18 years and it was a difficult decision. We've enjoyed a fantastic relationship and it's a very difficult time for us both."
Those 18 years encompassed Allardyce's playing and managerial career at the club.
He was interviewed for the England managerial position following Sven-Goran Eriksson's departure after the 2006 World Cup but was beaten to the job by Steve McClaren.
606: DEBATE
He pledged to focus on Bolton after missing out on a role he publicly stated he would "love", but steps down with two matches of the current campaign still to play.
The Trotters are currently in fifth spot in the Premiership and are chasing Uefa Cup qualification.
"It was mutually decided that I step down now rather than at the end of the season," revealed Allardyce in a statement on the club website.
"That will give my successor the ideal opportunity to experience the preparation and build-up of match days in readiness for next season.
"We owe it to Sam to make sure we get back into Europe. We will do our utmost to qualify for him"
"After guiding the team into a position where the club is on the verge of qualifying for European football for the second time in three years, I feel it is the right moment to step down.
"I feel the club is in a great position to continue its upward trajectory, with a talented squad supported by an excellent Academy programme under the guidance of a highly-skilled management team.
"The foundations for the next phase are well and truly in place. It is with this knowledge that I feel confident that I am moving on with my life, content with a legacy that I have tried to create for this special club.
"I wish the owner, chairman, staff, players and all the fans every success in the future."
BBC Sport columnist Kevin Nolan, who was handed his debut by Allardyce in 2000 and was eventually made club captain, said on the Bolton website: "I'm devastated and completely shocked by the news.
"However, as disappointing as the news is, there are only fantastic times ahead for this club. We've got a terrific club and talented squad who want to achieve things.
"He has helped design and build an infrastructure that will enable the club to continue the progress we have undoubtedly made during his period of office"
"I've got nothing but praise and respect for Sam. He has made me the player I am and has built this club.
"Now we owe it to him to make sure we get back into Europe. We will do our utmost to qualify for him."
BBC pundit Garth Crooks was equally surprised by the news of Allardyce's resignation.
"I'm stunned. I can't understand why he's gone after he's had such a great season," said the former Spurs striker.
"I also can't understand why the board accepted his resignation. Maybe there is more to it.
"There will be a long list of candidates to take over at Bolton, although Sam has constructed that squad over a long time and it will be difficult for the new man to come in and pick up where he left off."
Bolton coaches Sammy Lee and Ricky Sbragia are the early favourites to take over, at least until the end of the season, with former Fulham manager Chris Coleman and Wigan boss Paul Jewell other names touted by bookmakers.
Gartside said he had "reluctantly accepted" Allardyce's resignation after "eight years of unprecedented success at the club".
"Since joining the club as manager in 1999, he has helped lead a fantastic transformation of this famous Lancashire club," he added.
"He has helped design and build an infrastructure that will enable the club to continue the progress we have undoubtedly made during his period of office."