Derby manager Billy Davies has left the Premier League strugglers after talks with chairman Adam Pearson.
Davies has been in charge at Pride Park for 18 months
The club are two points adrift at the foot of the table following a 2-0 defeat by Chelsea on Saturday.
"We have decided mutually in the best interest of both parties that to go our separate ways is the correct decision at this time," said Pearson.
Academy boss Kevin Thelwell takes charge for Saturday's match with fellow relegation battlers Sunderland.
Derby have just one win so far this season, which came against Newcastle, and have conceded 33 goals.
The side have struggled to come to terms with the top flight after Davies, who joined in June 2006, steered them to promotion via the play-offs last season.
The former Preston boss had his fate sealed at a meeting on Monday with Pearson but did not help his cause with comments made after the loss to Chelsea as he hinted at frustration with the club's board.
He said: "It would be nice if I could have five minutes with him [Adam Pearson]. But I can't, he is a very busy man. I haven't spoken to him for two or three weeks."
Pearson, who has been working on attracting new investment to the club, said the board had lost patience with Davies because of his negative comments.
"I was surprised on Saturday night when it was suggested I hadn't spoken to Billy for three weeks because that is totally inaccurate," he told BBC Radio Derby.
"My last formal meeting 10 days ago was to ask for a list of players for Christmas and there have been subsequent phone calls so that caused a little bit of tension.
"Things were brought to a head because there were continual comments about lack of investment and that is hard for this club to accept because the wage bill's been increased significantly, £12m has been spent on players and the debt has been reduced by £6m."
There have been lots of ups and downs but in the main I'm proud of what has been achieved at the club
Davies, who has become the fifth managerial casualty in the Premier League so far this season, put a question mark over his future back in May after steering Derby to promotion.
But he signed a contract extension until June 2010 after talks with the board and ex-chairman Peter Gadsby, although that was another controversial episode in his tenure at the club.
Davies did not shed any light on any row with the Derby board.
He said: "I would like to thank the staff, players, wonderful fans of Derby County and the board for what has been a very enjoyable time for me.
"What's quite ironic on the morning of me parting company with Derby is that I am going to the [BBC] East Midlands [Sports] awards tonight to pick up the coach of the year award.
"There have been lots of ups and downs but in the main I'm proud of what has been achieved at the club and I walk away very much with my head held high."
Thelwell, who joined Derby as the club's academy boss in July 2006, will be assisted by goalkeeping coach Alan Fettis and senior squad members Matt Oakley and Darren Moore.
After meeting Sunderland at the Stadium of Light, Derby travel to Old Trafford to face champions Manchester United before a home match with Middlesbrough.