Gannon had been a controversial figure since his arrival in Scotland
Motherwell have sacked manager Jim Gannon, saying that the 41-year-old "repeatedly refused to sign a contract" with the Scottish Premier League club.
Gannon, previously of Stockport County, had been appointed on 30 June.
But a club statement said: "Relations between the manager, staff and directors deteriorated to the extent that the situation became untenable."
Motherwell claim it had nothing to do with recent results but say Gannon was "not fully committed to the club".
Gannon, who has the right of appeal against his dismissal, was believed to be consulting a solicitor on Monday afternoon.
Recently linked with the managerial vacancy at Sheffield Wednesday, Gannon has been a controversial figure since his arrival at Fir Park.
It is deeply regrettable that the board has felt it necessary to part company with the manager at this stage of the season
Motherwell chairman John Boyle
He was critical of captain and former caretaker manager Stephen Craigan's defending during Motherwell's humiliating 1-0 defeat by Welsh side Llanelli in their opening fixture of the season.
Although they recovered from that first-leg loss to progress in the Europa League qualifying rounds, his relations with Craigan never recovered and the Northern Ireland defender consistently found himself out of the startling line-up.
Gannon was also publicly critical of refereeing standards in Scotland and had a long drawn-out spat in the media with the Scottish Football Association's chief of referees Hugh Dallas.
He briefly refused to speak to the media because he was unhappy with reporting of his disagreement with Craigan and was launched his latest criticism of his players after Saturday's 3-1 defeat at home to St Johnstone.
However, Motherwell say it was the uncertainty over his tenure at Fir Park that led to their decision.
Gannon spoke publicly about his long-term plans for the club, but club sources suggest that he raised a different issue each time the papers were presented.
Improvements were made to his terms, including allowances for travel to and from his Cheshire home, but the contract remained unsigned.
"It is deeply regrettable that the board has felt it necessary to part company with the manager at this stage of the season, but we have reached a situation where we felt there was no other option," Motherwell chairman John Boyle said in a statement.
"Generally, results on the field have been satisfactory since Jim Gannon was appointed and the board wishes to make it clear that today's decision is not related in any way whatsoever to recent results.
"Over the past couple of months, it has become abundantly clear that the manager was not fully committed to the club and, despite our best efforts, he repeatedly refused to sign a contract."
Motherwell's head of youth Gordon Young will take charge of the team for Wednesday's SPL match against Hearts at Tynecastle.
They go into that game in sixth spot having not won in their last seven league outings.
Gannon, who had led Stockport to promotion to League One but was made redundant in May as the club went into administration, had taken charge at Fir Park after the departure of Mark McGhee to Aberdeen.
He inherited a squad minus several experienced summer departures and bolstered it with players he knew from England's lower divisions, plus two on loan from Everton, he thought could make an impact in Scotland's top flight.
However, it was Motherwell's emerging young players who impressed at the start of the league season, but on Saturday his frustration at their inconsistency led Gaggon to say he did not have "enough players with physical and mental strength who can keep clean sheets".
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