A backbench MP has revealed a Tory plot to oust David Cameron, confirming he was asked to consider a "stalking horse" leadership challenge to the prime minister.

Colonel Bob Stewart said he was approached by two party colleagues before the summer recess with the proposal but that he told them to "get lost" and rejected the idea as "silly".

He declined to name the plotters and said he had not been told on whose behalf the pair were operating.

Details of the scheming was exposed by the Mail yesterday, which also reported that Tory MP Zac Goldsmith had offered his safe Tory seat to Boris Johnson.

Goldsmith has vowed to resign as the MP for Richmond in south-west London if the government revives controversial plans for a third runway at Heathrow airport which the mayor of London also fiercely opposes.

The newspaper said that the idea of Johnson fighting the subsequent byelection to secure a return to the Commons and the chance to challenge for the leadership was made in talks to discuss how best to oppose the airport expansion.

The idea was said to have been dismissed "out of hand" by the mayor, whose position as a potential challenger to Cameron has been reinforced by the success of the London Olympics.

The mayor's official spokesman said: "It's no surprise that the mayor has met and will continue to meet those who share his concerns over the lack of aviation capacity in London and the south-east.

"But the story in question is without doubt fanciful."

Johnson, who has dismissed suggestions he could cut short his mayoral term to launch a leadership bid, has led the outcry over the reconsideration of Heathrow expansion.