Boris Johnson has declined to rule out the possibility of a future bid to become Conservative leader and Prime Minister - but insisted he supports David Cameron and described rumours about his ambitions as "a distraction" from his job as mayor of London.
Challenged to say that Mr Cameron was a better man than him to be PM, Mr Johnson would say only that it was "unverifiable". He said that "for the next few years" he only wanted to be mayor, but added that "heaven knows" what he will do after his four-year term ends in 2016.
In an interview for Pienaar's Politics on BBC Radio 5 Live, Mr Johnson conceded that it was "entirely natural" for the media to discuss a narrative in which he was considered as being in competition with the PM.
Speculation that Mr Johnson has his eye on Mr Cameron's job is dominating the Conservative Party conference in Birmingham, after a poll suggested voters prefer the London mayor to Mr Cameron by a wide margin. The survey by pollsters Opinium for The Observer gave Mr Johnson a net +30 rating compared to -21 for the Prime Minister.
Mr Cameron insisted he did not feel threatened by the mayor's popularity, and said he wanted Mr Johnson to move on to further top-level political jobs after he has finished his stint at City Hall.
"I have got the opposite of tall poppy syndrome," the PM told BBC1's Andrew Marr Show. "I like having other people in the Conservative Party who are popular, who get out there, talk our message and explain our vision and values.
"I have said to Boris: 'Once you have done your job as London mayor, don't think your job in politics is over.' I think he has got a huge amount to offer, a huge amount to give and I encourage him to do that."
Mr Johnson told interviewer John Pienaar that he "despaired" of the constant gossip about his position. But asked whether Mr Cameron was a better Prime Minister than he could be, the London mayor answered: "I think it's unverifiable, but I think he's doing a fantastic job."
He added: "My function is to run London, I have been elected to do that, and David Cameron is overwhelmingly the best man now to lead this country, to clear up the mess that Labour left, and I think he is doing a bang-on job."
Challenged directly over whether he had ambitions to be PM in future, Mr Johnson replied: "Do I want to be Prime Minister? No, I want to be mayor of London, certainly for the next few years." The 48-year-old added: "After four years are up, heaven knows. I will be an old man."