Newcastle have ended Sam Allardyce's managerial reign after eight months.
The club, currently 11th in the Premier League, said the decision had been reached "by mutual agreement".
Allardyce, who was in charge for 24 games, said: "I'm disappointed but I wish them all the best for the future. It was a shock, I didn't expect it."
Former Magpies captain Alan Shearer is extremely unlikely to take over, with chairman Chris Mort revealing: "A new manager has not yet been appointed."
First-team coach Nigel Pearson will take charge of the team for Saturday's Premier League meeting with Manchester United.
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Earlier on Wednesday, Allardyce had attended a news conference ahead of that game but gave no hint that his time at the club might be coming to an end.
Indeed, the 53-year-old had discussed his plans for the January transfer window and, ironically, spoke about the lack of time afforded to Premier League managers in the modern-day game.
However, shortly afterwards, Newcastle announced news of his departure with a statement on their official website.
Allardyce signed a three-year deal when he replaced Glenn Roeder in May after eight years in charge at Bolton.
He was quickly working under a new regime at St James' Park after billionaire Mike Ashley completed his takeover, and long-standing chairman Freddy Shepherd left the club.
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The new manager signed nine players during the summer at a total investment of around £26m, with Joey Barton (£5.8m), David Rozehnal (£2.9m) and Alan Smith (£6m) among the new arrivals.
The Magpies began the season with a 3-1 win at the Reebok Stadium but he soon came under fire from fans unhappy with the team's style of play.
The pressure built with a run of three straight league defeats over the festive period - although midfielder Nicky Butt was among those who urged the club to keep faith with Allardyce.
LISTEN: Comment and reaction on BBC Tyne
"It was an open secret that he was under pressure after making such a mediocre start - but the timing of his leaving was still unexpected," said BBC Radio 5 Live's chief football correspondent Mike Ingham.
"But Newcastle's problems stretch back over 30 years - not half a season. Managers have been hired and fired far too quickly.
"When will the penny finally drop that stability and continuity are the main ingredients needed to achieve success?"
"The pressure is too much now in the Premier League"
Former Newcastle boss Sir Bobby Robson said the news of Allardyce's departure had come as a great shock to him.
"I was very surprised and stunned when I heard the news," Robson told BBC Radio 5 Live.
"That's the eighth Premier League manager we've lost this season already. The pressure is too much now in the Premier League.
"The dismissals of manager are coming fast and frequent and it's rather sad for the game in many respects."
Allardyce's last game in charge was a 0-0 draw at Championship side Stoke in the FA Cup third round on Sunday.
He becomes the eighth Premier League managerial casualty this season, leaving with an overall record of eight wins, six draws and 10 defeats.
Since Kevin Keegan left 11 years ago, six permanent managers have now come and gone at St James' Park, while it has been 39 years since the club claimed a major trophy.
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