Darrell Clarke has left his role as manager of Bristol Rovers after four and a half years in charge.
Rovers have lost seven of their past 10 games in all competitions and are 21st in League One, four points from safety.
Coach Graham Coughlan will take interim charge for Saturday's game at Sunderland but assistant manager Marcus Stewart has left by mutual consent.
Having overseen their relegation from the EFL in 2014, Clarke guided Rovers to successive promotions to League One.
But after Saturday's loss to Doncaster the 40-year-old told BBC Radio Bristol he was "under-achieving as a manager" and that they were "going backwards".
"After much soul-searching and lengthy deliberations it has been mutually agreed that it is probably in the best interests of the club and of Darrell Clarke to seek a fresh way forward," said a Bristol Rovers statement.
"Darrell will remain a friend of the club and will no doubt achieve further success in his career after a deserved break.
"The board is expecting substantial interest in the vacant manager's role and an appointment will be made as soon as practical."
A further statement added that outgoing number two Stewart had "asked to be released from his contract" and the club had "reluctantly agreed".
From relegation to back-to-back promotions
Having played for Mansfield and Hartlepool, Clarke joined Rovers as assistant manager to John Ward in June 2013 after a stint in charge of non-league Salisbury City.
In March 2014 he replaced Ward as manager, but could not stop the club from dropping out of the EFL for the first time in 94 years as they lost 1-0 to Mansfield on the final day of the season, despite only needing a draw to survive.
They returned to League Two at the first attempt, finishing a point behind Conference winners Barnet before going on to beat Grimsby on penalties in the promotion final at Wembley.
Another successful season followed as Rovers beat Dagenham & Redbridge 2-1 on the final day of the campaign to secure third place in League Two on goal difference and back-to-back promotions.
In their first season under Clarke in the third tier they finished 10th, six points off the play-offs, while last season they were 13th.
This term Rovers have won just four of their 21 league fixtures and were knocked out of the FA Cup in the first round by National League side Barnet at the Memorial Stadium last month.
Richard Hoskin, Sports Editor BBC Radio Bristol
Darrell Clarke was the fourth longest-serving manager in the EFL and was also one of the most popular managers in Bristol Rovers' recent history, winning back-to-back promotions from the Conference to League One.
This season, though, he has cut a frustrated figure. After Saturday's 4-0 home defeat against Doncaster, he said the club was going "backwards", warning of cuts. Clarke also said he didn't think a new stadium or training ground would be built while he was in charge.
By his own admission, mistakes have been made. Summer signings have been disappointing, and Rovers have struggled to score goals.
Now eyes turn to the owners, with questions remaining about the future direction of the football club. Bristol Rovers are also in the League One relegation zone, four points from safety ahead of a daunting trip to Sunderland this weekend.