The 55-year-old Portuguese’s last match in charge was the 3-1 defeat by Premier League league leaders Liverpool on Sunday which left them 19 points behind their opponents.
Mourinho’s sacking comes just two-and-a-half years into his tenure and three years and a day since Chelsea sacked him just months after he had guided them to the title.
Former player Michael Carrick, now a member of the coaching staff, is expected to take charge in an interim role but United said a caretaker manager will be appointed until the end of the season.
The club’s worst start in the league since 1990 means even Mourinho’s ambition of reaching the top four and a Champions League qualification spot is in danger.
Despite United reaching the Champions League knockout stages — they face an intimidating last-16 tie with Paris Saint-Germain — a mix of the results, dressing-room discord and criticism of the board’s transfer policy proved potent factors in the decision to dismiss him.
A United source told AFP that Mourinho had been supported in his efforts to sign players and described reports of player power at the club as being key to the sacking as being wide of the mark.
According to British media reports, United will have to pay £22.5 million ($28 million) — as the club are still in the Champions League — to sever the ties.
“Manchester United announces that manager Jose Mourinho has left the club with immediate effect,” a club statement said.
“The club would like to thank Jose for his work during his time at Manchester United and to wish him success in the future.
“A new caretaker manager will be appointed until the end of the current season, while the club conducts a thorough recruitment process for a new, full-time manager.”
Carrick is favoured for the caretaker role having become part of the coaching staff this season.
– ‘Deteriorating relationship’ –
In the longer term former Real Madrid coach Zinedine Zidane has been linked to the club and Tottenham Hotspur manager Mauricio Pochettino could also be a name in the frame.
Mourinho’s reign had started well enough with the League Cup and the Europa League trophies but neighbours Manchester City’s dominance over them in the league has hurt.
The wound went even deeper for Mourinho as City are managed by Pep Guardiola, who got the better of him when he managed Barcelona and Mourinho was at Real Madrid.
However, despite his protestations to the contrary the United board gave Mourinho plenty of money to compete.
Top-quality players such as Belgian striker Romelu Lukaku — who he persuaded to come to United and not Chelsea for £75 million in 2017 — and Paul Pogba for a then world record fee of £89 million in 2016 have floundered under his authoritarian stewardship.
Indeed it was his deteriorating relationship with World Cup-winning midfielder Pogba — one of many to feel the lash of his tongue in public — that symbolised the decline at the club under him.
Mourinho questioned his attitude while the player hit back with open criticism of the tactics, culminating in him kicking his heels on the bench during the Liverpool debacle on Sunday.
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