Stuart McCall has quit as Motherwell manager with the Lanarkshire club just two points above the automatic relegation place.
Assistant manager Kenny Black has been named caretaker manager until a successor has been appointed.
Director Derek Weir, speaking on behalf of the board, said: “Everyone at Motherwell FC would like to sincerely thank Stuart for his contribution to the club, which will go down as one of the most successful managerial tenures in our 128-year history.
“We have been acutely aware of results this season, but everyone at Fir Park was confident in his, and the team’s, ability to turn things around.
“However, following a long conversation this afternoon, Stuart felt it better for all concerned if he stepped down with immediate effect and we respect that.
“Assistant manager Kenny Black will take charge of first-team team affairs in a caretaker role to allow the board to start the process of recruiting a new manager, which will begin right away.”
McCall joined Motherwell in 2010, leading them to the 2011 Scottish Cup Final and finished in the top three in the last three seasons.
The former Rangers player also profited from his former employers’ demise, when Motherwell entered into the 2012 Champions League qualifiers losing to Panathinaikos.
Former Motherwell managers Mark McGhee and Terry Butcher, Gary McAllister, Derek Adams, Ian Murray, Owen Coyle and Scott Leitch are among the names linked with the vacant position.