A Tory plot to oust David Cameron has been revealed when a backbench MP confirmed he was asked to consider a “stalking horse” leadership challenge against the prime minister.

Colonel Bob Stewart said he had been 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”.

Details of the scheming were exposed by the Mail On Sunday newspaper, which also reported that Tory MP Zac Goldsmith had offered his safe Tory seat to London’s mayor Boris Johnson.

Goldsmith has vowed to resign as the MP for Richmond in southwest London if the government revives controversial plans for a third runway at Heathrow airport – which is also fiercely opposed by the Johnson.

The newspaper said that the idea of Johnson fighting the subsequent by-election to secure a return to the House of 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 Johnson, whose position as a potential challenger to Cameron has been reinforced by the successful London Olympics.

Asked about the separate plot, Stewart told the Press Association: “It was a silly suggestion that I dismissed. I told them to get lost.

“I was not having anything to do with it.”

He declined to name the plotters and said he had not been told on whose behalf the pair were operating: “I do not know; I do not care.”

He insisted he remained loyal to Cameron, who was “the reason I came into politics”.

“I may have rebelled by voting against House of Lords reform and on Europe but that does not make me disloyal. He is the leader of our country, the leader of our party and I believe in loyalty.”