Sam Allardyce resigned as manager of Premier League club Bolton Wanderers with immediate effect yesterday but the reason for his decision to leave after eight years remained unclear.

Club chairman Phil Gartside said: "He has given his reasons for wanting to leave the club at this time, which are private."

Allardyce said it was mutually decided that he would leave now, rather than the end of the season, which is another successful one for Bolton who are currently fifth and on course for a UEFA Cup place.

Some newspapers have linked Allardyce with Manchester City, who are involved in takeover talks, but Allardyce, who lost out to Steve McClaren for the England manager's job last year, made no reference to any other position.

This season he has had a bumpy relationship with the media after the BBC's Panorama programme accused him of accepting illegal payments related to transfers. Allardyce denied the claim and has refused to speak to the BBC for most of the season as a result.

Allardyce, an uncompromising central defender with Bolton in his playing days, took over from Colin Todd in 1999 and won promotion to the Premier League two years later.

After narrowly avoiding relegation in his first two seasons, Bolton then became established as a major Premier League team finishing eighth, sixth, and eighth. They also reached the League Cup final in 2004.

Allardyce specialised in getting the best out of "difficult" players who other clubs struggled with, while he was in the forefront of the appliance of scientific methods to training and diet.

His squads were among the most multi-national in the league while his teams' no-nonsense approach meant Bolton became feared opponents.

On Saturday they came back to draw 2-2 with Chelsea at Stamford Bridge, a result that severely weakened Chelsea's grip on the title.