Alan Pardew has been sacked as manager of West Ham.
The Hammers are in 18th place in the Premiership following a run of three straight defeats that culminated in Saturday's 4-0 thrashing at Bolton.
"Alan has made an important contribution since joining in September 2003 but this season's results have been disappointing," said a statement.
"Chairman Eggert Magnusson and the board feel it is the right time to make a change in the club's best interests."
The statement continued: "The search for a successor is now under way in order that a new manager can be in place ahead of the January transfer window.
"The supporters have stood by me and I will always cherish memories of their loyalty"
"First-team coach Kevin Keen will take temporary charge of team affairs until a new appointment has been made.
"No further statements on this matter will be made by the club before that time."
Former Charlton boss Alan Curbishley, who played for the Hammers from 1975 to 1979, has been installed as the early favourite to take over as manager.
Pardew told the club website he was proud of his achievements with the club.
"In two consecutive seasons we qualified for the play-offs, ultimately achieving the Premiership status the club so richly deserve," he told the West Ham website.
PARDEW'S WEST HAM RECORD
"I am also proud of our first season back in the top flight in which we finished ninth and came so close to winning the FA Cup.
"The supporters have stood by me and I will always cherish memories of their support and loyalty during the difficulties of this season.
"I am proud most of all of the players, many of whom have developed into top Premiership performers under the tutelage of myself and my backroom staff.
"The coaching and medical staff at Upton Park are second to none and I want to wish the new chairman Eggert Magnusson and the club every success for the future."
Following a disappointing start to the season, the Hammers were briefly boosted when a consortium led by Magnusson completed a £85m takeover, with the team winning their next match against Sheffield United.
606: DEBATE
Magnusson initially vowed to keep faith with Pardew, promising funds for the January transfer window.
However, the club's subsequent slide into the bottom three forced the Icelander's hand and Pardew is the second managerial casualty of the Premiership season - following Iain Dowie's exit from Charlton.
After a shaky start, Pardew became a firm favourite at Upton Park as he steered them to promotion to the Premiership in 2005 and then cemented their position in the top flight by guiding them to the FA Cup final and a Uefa Cup place.
However, it has been an entirely different scenario this term, with the Hammers finding points hard to come by - despite the high-profile signings of Argentine internationals Carlos Tevez and Javier Mascherano.
THE RACE TO SUCCEED PARDEW
Hammers legend Frank Lampard Snr believes the arrival of the Argentine duo at the end of August was central to the club's demise.
"It was only six months ago that everyone was saying how well Alan was doing," Lampard told BBC Five Live.
"It seems since he brought in Tevez and Mascherano it's all gone wrong.
"It's a shame that he's gone but football is like that. You get your chance and as soon as you start losing games, the pressure's on."
Manchester United boss Sir Alex Ferguson expressed his sympathy for Pardew.
"Football is unfortunate in that it is a kind of industry where failure is quickly decided upon. I'm sorry to hear it, I don't like anyone losing their job," he said.