Chelsea have sacked manager Jose Mourinho seven months after he led them to the Premier League title.
The 52-year-old Portuguese had been in his second spell at the club, taking charge in June 2013.
Chelsea finished eight points clear last season and won the League Cup, but have lost nine of their 16 league games so far and are 16th in the table, one point above the relegation places.
Mourinho's final match was Monday's 2-1 defeat at leaders Leicester City.
Pep Guardiola, Guus Hiddink, Juande Ramos and Brendan Rodgers have all been touted as possible successors as Blues owner Roman Abramovich begins another managerial search.
"Chelsea Football Club and Jose Mourinho have today parted company by mutual consent.
"All at Chelsea thank Jose for his immense contribution since he returned as manager in the summer of 2013.
"His three league titles, FA Cup, Community Shield and three League Cup wins over two spells make him the most successful manager in our 110-year history.
"But both Jose and the board agreed results have not been good enough this season and believe it is in the best interests of both parties to go our separate ways.
"The club wishes to make clear Jose leaves us on good terms and will always remain a much-loved, respected and significant figure at Chelsea.
"His legacy at Stamford Bridge and in England has long been guaranteed and he will always be warmly welcomed back to Stamford Bridge.
"The club's focus is now on ensuring our talented squad reaches its potential.
"There will be no further comment until a new appointment is made."
BBC Match of the Day pundit Alan Shearer said he was "shocked" by Mourinho's departure but admitted Chelsea's form had been dreadful.
"I've never known a capitulation like it from a football club," the former England captain told BBC Radio 5 live.
"I have never known players to perform like they did last season and then be so bad now. It's unprecedented."
Sunderland boss Sam Allardyce said he was "shocked to say the least", adding: "They must already have somebody lined up to have done it now.
"I'm sad to see Jose go because I got to know him quite well and he's a great manager with great character. He's a loss to the Premier League."
Former England and Arsenal defender Martin Keown said Mourinho had "lost the players".
"It has imploded in front of our eyes," he said. "It is astonishing. I saw players who were not giving everything.
"There is no trust there, respect is gone. Ultimately it is easier to change a manager than 22 players."
Ex-Blues defender Graham Roberts told BBC Radio 5 live: "He came in and said he was 'The Special One'. He's not 'The Special One' any more."
"Jose Mourinho had a meeting with the club at around 14:30 GMT, when he was told the end had come.
"He won't be paid the £40m compensation speculated. My understanding is he has been paid until the end of this season, what was left of the annual salary of £12m he receives.
"While he has undeniably enjoyed so much success around Europe, he has failed to secure a lasting legacy and enjoy success over a prolonged period of time.
"Chelsea faced a huge decision and Roman Abramovich had to make it."
Mourinho leaves with Chelsea only a point above the relegation zone and 20 points behind Leicester.
Champions League qualification via one of the top four spots in the Premier League looks highly unlikely.
However, the Blues are through to the last 16 of European football's biggest club competition, where they will face French champions Paris St-Germain in February and March.
Mourinho only signed a new four-year contract on 7 August and is Chelsea's most successful manager.
He won three Premier League titles, two of those in his first spell between 2004 and 2007.
The Portuguese has also won the FA Cup, in 2007, and the League Cup three times - in 2005, 2007 and 2015.
Mourinho celebrates another Premier League title last season
It has not only gone wrong in the Premier League for Mourinho; he has also had his issues off the pitch.
A 2-2 draw with Swansea on 8 August was overshadowed by a fallout with his medical team, in particular club doctor Eva Carneiro.
Carneiro had her role downgraded after Mourinho said his medical staff were "naive" for running onto the pitch to treat midfielder Eden Hazard.
She eventually left on 22 September and is now taking legal action against Mourinho and the Blues.
Mourinho also received a suspended stadium ban and £50,000 fine forclaiming referees were "afraid" to award his team penalties in a 3-1 home loss to Southampton in October.
He subsequently had to serve a one-match stadium ban and pay a £40,000 fine for his behaviour during a 2-1 defeat by West Ham, when he spoke to referee Jon Moss at half-time.
Jose Mourinho watches on as doctor Eva Carneiro runs onto the pitch at the Swansea game in August
It is unlikely he will be out of management for long, given his record of success at Chelsea and other top European clubs.
He guided Porto and Inter Milan to Champions League glory in 2004 and 2010 respectively, then Real Madrid to the Spanish La Liga title in 2012.
However, Shearer said his departure from Chelsea "will tarnish him".
At 52, Mourinho is still some way from retirement age, but he has expressed a wish to coach the English and Portuguese national sides at some stage in his career.
In fact, he claimed he turned down the chance to become England boss when Steve McClaren was sacked in 2007.
Mourinho has a proven track record of bringing success over the short term but doubts will remain over his ability to build a dynasty.
Will that make his candidature less appealing to the kind of club he is used to managing, such as Bayern Munich, Paris St-Germain, Manchester United or Manchester City?
The Blues may be struggling at the wrong end of the Premier League table, but they are still in the Champions League and the FA Cup, while achieving European qualification is not impossible.
The key for any new manager is improving the morale of a talented squad that has underachieved this season.
Players such as Eden Hazard, Diego Costa, Willian, and Oscar were key to Chelsea winning the league title but performances have fallen well below the heights they reached last year.
After the 2-1 defeat by Leicester Mourinho said he felt his "work was betrayed" by his players, and midfielder Cesc Fabregas has said it is time for everyone to step up.
"If you are a big player and paid like a big player, you must play like a big player and behave like a big player," said the Spain international.
Short term, it's looking like ex-Netherlands coach Guus Hiddink, who has already had a stint in charge of the Blues, and former Tottenham Hotspur and Sevilla manager Juande Ramos.
Both are without a job at the moment so are able to slot in quickly.
But will either be in charge beyond the end of the season, when the likes of Pep Guardiola might be available?
The former Barcelona boss is due to make an announcement on his Bayern Munich future next week.
Speculation is he will quit the Bundesliga giants when his contract finishes at the end of the current campaign.
Other likely candidates are Atletico Madrid's Diego Simeone, Carlo Ancelotti, who guided Chelsea to the Premier League and FA Cup double in 2010, and former England boss Fabio Capello.