Manager Karl Robinson has left MK Dons "by mutual consent", with the club one point and two places above the League One relegation zone after 15 games.
Robinson, 36, has been in charge at the Dons since May 2010, winning promotion to the Championship in May 2015, but was unable to maintain their second-tier status the following season.
Saturday's 3-0 home defeat by Southend extended the winless run to four games.
Head of coaching Richie Barker will take temporary charge of the team.
Robinson was the third longest-serving manager in all four divisions, with only Arsenal boss Arsene Wenger and Exeter manager Paul Tisdale having been in post longer.
The Liverpudlian said in his final post-match interview that he would not walk away from the job, but that it was chairman Pete Winkleman's decision.
"Robinson fully embraced and immersed himself in MK Dons and Milton Keynes itself, which was reflected in his close relationship with staff at the club and its supporters," the club's statement said.
"However, the start to the League One campaign has been disappointing with results this season not matching the desired standard. Therefore, chairman Pete Winkelman and Robinson have agreed it is in the best interest of both parties to go their separate ways."
A man in demand?
Although Robinson's relations with Winkelman, the players and fans was a strong one, there was some discussion from the chairman that he could have left the club in the summer, post-relegation.
But he turned down a move to Leeds United following Massimo Cellino's approach in a bid to steer the Dons back to the Championship.
"I share everyone's frustrations, whether it be the chairman or the fans or my family. Everyone is in a low place," Robinson said after the Southend loss on Saturday.
"Certain players are too weak - mentally and physically too weak. It affects my life, but I don't think it affects certain players' lives. You have to be responsible for your own actions as well. It is not all about the players.
"Me and the chairman will have a chat on Sunday. If it is to be [leaving the club] then I want to say thank you for everyone who has supported me over the last six and a half years."
From Marine to MK
Robinson's appointment as MK Dons boss made him the youngest manager in the Football League at the time, aged 29.
His playing career was spent exclusively in non-league, but he built a reputation as a youth coach at Liverpool and then Blackburn, where he worked with Paul Ince, who later brought him to the Dons.
That youth development work was continued at MK, with players such as Dele Alli, Brendan Galloway, Patrick Bamford and Benik Afobe going on to play Premier League football after their stints in Buckinghamshire.