The relationship between the Portuguese manager and club owner Roman Abramovich has been strained for several months, with Abramovich apparently wanting a more spectacular style of football and Mourinho angered at having players foisted upon him.
Mourinho is also thought to have been unhappy that the Israeli coach Avram Grant was appointed director of football in the summer, a role that undermines his authority.
Chelsea have begun the season disappointingly, scraping a draw at Liverpool, losing at Aston Villa and then drawing against Blackburn last Saturday.
The final straw seems to have come Tuesday, as Chelsea drew with Rosenborg in the Champions League in front of a Stamford Bridge crowd of under 25,000 - their lowest in four years.
Mourinho, who still has three years left on his contract, led the club to two Barclays Premier League titles after joining in June 2004 but their disastrous start to the new season had taken its toll on him.
Chelsea face Premiership champions Manchester United Sunday.