Conservative party chairman Grant Shapps has laid down the gauntlet to London Mayor Boris Johnson, with what press commentators have described as a "surreal" disclaimer of his political ambitions.

Johnson, widely expected to launch a bid for the party leadership, has repeatedly asserted that he has no ambition to be prime minister.

The London mayor was put on the spot on Sunday 24 March by BBC journalist Eddie Mair, who suggested he was a "nasty piece of work" because he deceived then Tory leader Michael Howard over an extra-marital affair, among other things.

Shapps told the Sunday Telegraph: "The average human life span is 80 to 100 years each, while the planet's been around for billions of years - the universe for 14 billion."

He added: "My great hope for my speck of time on this planet would be that I live and die, and that what I leave behind has made some kind of impression, and has been for the better in terms of my family and friends and, in my case, public service."

Interviewed on the BBC's Andrew Marr show by presenter Eddie Mair, Johnson was asked directly of his leadership ambitions. Johnson said: "Look, what I want is to do as well as I can as Mayor of London. I think people will rightly conclude that I don't want to talk about this subject, because I want to talk about what I think should happen, which is the government should win the next election."

Shapps served as housing minister before replacing Sayeeda Warsi as Conservative party chairman in David Cameron's reshuffle last year.

Johnson had attempted to play down his leadership ambitions in recent years, comparing the odds to that of being "reincarnated as an olive".

"All I want is for David Cameron to win this election. I want to do everything I can to help," Johnson said. "People will rightly conclude I don't want to talk about this subject."

Shapps was lampooned when it emerged he had employed the alias Michael Green as part of a get-rich-quick internet scheme in which he asked respondents to participate in his "20/20 Challenge e-Book", and promised subscribers they could "make $20,000 in 20 days" by following his "toolkit".

"That's what I want to do and I'm perfectly happy doing it in the sort of roles that have come along so far, as a minister and party chairman and in the Cabinet," Shapps told the Sunday Telegraph.

"I don't spend lots of time thinking about what I shall do in the future."