Manager Claudio Ranieri has been sacked by Leicester City, nine months after leading them to the Premier League title.
The Foxes are one point above the relegation zone with 13 matches left.
"The board reluctantly feels that a change of leadership, while admittedly painful, is necessary in the club's greatest interest," read a statement.
Ranieri, 65, guided the Foxes to the title despite them being rated 5,000-1 shots at the start of the campaign.
- Analysis: Were Leicester right to sack Ranieri?
- Ranieri sacking unforgivable - Lineker
- Reaction to Ranieri's sacking
- Listen to 5 live's Football Daily
- 'One of the worst things the owners have done'
His departure comes less than 24 hours after a 2-1 defeat at Spanish side Sevilla in the first leg of their Champions League last-16 tie.
The Foxes took the Premier League title by 10 points but have won just five top-flight games this season, and could become the first defending champions to be relegated since 1938.
They have lost their past five league matches and are the only side in the top four English divisions without a league goal in 2017.
In a BBC Sport poll, 87% of voters think it was wrong for Leicester to sack Ranieri.
|Leicester's past two seasons after 25 games|
"His status as the most successful Leicester City manager of all time is without question," added the Foxes statement.
"However, domestic results in the current campaign have placed the club's Premier League status under threat."
Earlier this month, Leicester gave their "unwavering support" to the Italian, who was appointed manager in July 2015 and signed a new four-year deal in August 2016.
"After all that Claudio Ranieri has done for Leicester City, to sack him now is inexplicable, unforgivable and gut-wrenchingly sad," said former Foxes striker Gary Lineker.
BBC sports editor Dan Roan said: "Ranieri was told he was sacked on Thursday afternoon in Leicester once the team returned from Spain, but the suggestion is the owners decided before that defeat by Sevilla.
"The decision was taken very reluctantly but the club's owners are desperate to avoid relegation and its consequences.
"A news conference with assistant manager Craig Shakespeare will take place at the stadium at 13:00 GMT on Friday."
'Club interests above sentiment'
Leicester City vice-chairman Aiyawatt Srivaddhanaprabha:
"This has been the most difficult decision we have had to make in nearly seven years since King Power took ownership of Leicester City. But we are duty-bound to put the club's long-term interests above all sense of personal sentiment, no matter how strong that might be.
"Claudio has brought outstanding qualities to his office. His skilful management, powers of motivation and measured approach have been reflective of the rich experience we always knew he would bring to Leicester City. His warmth, charm and charisma have helped transform perceptions of the club and develop its profile on a global scale. We will forever be grateful to him for what he has helped us to achieve.
"It was never our expectation that the extraordinary feats of last season should be replicated this season. Indeed, survival in the Premier League was our first and only target at the start of the campaign. But we are now faced with a fight to reach that objective and feel a change is necessary to maximise the opportunity presented by the final 13 games."
Leicester's next game in their fight for Premier League survival comes when they host Liverpool on Monday.
Assistant manager Craig Shakespeare and first-team coach Mike Stowell will take charge of the squad until a new manager is appointed.
Ranieri's compatriots Paolo Benetti and Andrea Azzalin, both key members of his coaching staff, have left the club.
"The board will now begin the recruitment process and will make no further comment until that process is completed," Leicester added.
'Dilly ding dilly game's gone' - social media reaction
Analysis - 'How can Claudio be gone?'
Phil McNulty, BBC chief football writer:
It is almost the thought that dare not speak its name amid the wave of shock, outrage and disgust at Leicester's decision to ruthlessly dismiss the hugely popular 65-year-old, who won the hearts of all supporters with his good humour, class and dignity as he led the Foxes to the title.
But is there actually method in what many see as the madness of the club's Thai owners?
Leicester's fall has been more dramatic than anything they could have foreseen in their worst nightmares. A win for any of Sunderland, Crystal Palace and Hull City this weekend would put the Foxes in the relegation places. Wins for all three and they would be bottom by the time they face Liverpool on what will now be a highly charged occasion at the King Power on Monday.
After 26 games last year they were top on 53 points, two ahead of Spurs. This season they are 17th after 25 games, with only 21 points. Last season they had lost only three games compared with 14 in this campaign, and conceded only 29 goals compared with 43 this term. Indeed, they only conceded 36 in the entire 2015-16 season.
The difference is stark and, very clearly in the opinion of Leicester's owners, dangerous.
Ian Stringer, BBC Radio Leicester:
I can't believe it. I was in Seville on Wednesday, they got an away goal and the fans were really happy on the streets after Jamie Vardy scored.
He's gone. How can Claudio be gone? The greatest achievement ever. They are not in the relegation zone, the stadium has been singing his name and it's been very loud.
I think there will be a split. He is a hero, they could build a statue of him in the next month and no-one would bat an eyelid, the greatest manager ever at this club. But is he the man to get you out of a relegation fight?
The chairman is a Thai businessman; a billionaire; a very successful man. He kept the faith with Nigel Pearson when he struggled but brand Leicester City is very different these days.
A big decision, a big call, but still surprising.
|Leicester City: Premier League 2015-16 v 2016-17|
|Pts per game||Shots per game||Clean sheets||Defeats|
|2015-16||2.1 (81 from 38)||13.8 (7th in league)||15||3 (from 38)|
|2016-17||0.8 (21 from 25)||10.8 (14th)||5||14 (from 25)|
A year (and nine days) in the life of Ranieri
14 February 2016: Leicester lose 2-1 at Arsenal, their final defeat of the 2015-16 season before a 12-game unbeaten run.
2 May 2016: The Foxes are crowned champions of England for the first time in their history as Tottenham draw at Chelsea.
16 July 2016: Midfielder N'Golo Kante leaves to sign a five-year deal with Chelsea.
13 August 2016: Leicester lose their first game of the 2016-17 season - a 2-1 defeat at Hull City.
15 October 2016: The Foxes are hammered 3-0 by table-topping Chelsea at Stamford Bridge.
22 November 2016: Leicester secure top spot in their Champions League group with one game to spare.
18 December 2016: Ranieri is named Coach of the Year at the BBC Sports Personality of the Year awards.
7 February 2017: After a run of two wins in 15 league games, Leicester give Ranieri their "unwavering support".
22 February 2017: The Foxes lose 2-1 to Sevilla in the first leg of their Champions League last-16 tie.
23 February 2017: Ranieri is sacked.