Phil Parkinson and his coaching staff have been sacked by Charlton's new owners.
The decision, announced on the club's official website, comes after the 4-2 defeat at home to Swindon yesterday and the League One club will announce the appointment of a caretaker manager "in due course".
Following the takeover of the club last week, new chairman Michael Slater had stated he would back Parkinson in the January transfer window and give him every support.
Slater has now backtracked on his comments, confirming the departure of Parkinson, assistant manager Tim Breacker and first-team coach Mark Kinsella.
"Clearly improvement is needed on the field. The team has not won in the league since November and recent performances have simply not been good enough," he said.
"Last night's defeat convinced us as a board that change is required now while we are still in the hunt for promotion, and that we must appoint a new manager to give us every chance of going up."
Parkinson was appointed at The Valley in 2008 and leaves the Addicks in fifth place in League One, just three points off the automatic promotion places.
In a statement released through the League Manager's Association, Parkinson said: "Naturally I am very disappointed to be leaving Charlton Athletic. This campaign has proved to be extremely competitive, with only a few points separating several clubs.
"I firmly believed in the players' abilities to improve on last season's finish and again compete for promotion back to the npower Championship.
"I would like to take this opportunity to wish the club, the management, the players and the supporters all the very best for the future. I now look forward to a new challenge in the game.''
Richard Bevan, the LMA chief executive who has seen several of his members sacked over the past few weeks, expressed disappointment at the trend, as well as surprise that Parkinson should have been dismissed despite Charlton's league position.
"It is extremely disappointing that Phil has become the next manager to get the sack in this new year period,'' he said. "It is even more remarkable given Charlton currently sit fifth in npower League One, only three points from the automatic promotion places.
"Phil was awarded the npower manager of the month award as recently as November, testament to the outstanding work he has done with his squad so far this season, following a summer that saw the turnover of over 30 players in his playing staff.
"Charlton are losing a young, talented manager, whom at the age of 43 has already managed over 300 games. Without a doubt Phil will take his knowledge and skills to another club where I am certain he will be a success. Phil's departure will be a big loss to Charlton Athletic Football club, its players and its fans.''