Mayor of London should not be Boris Johnson's last job in politics, Prime Minister David Cameron has said.

Mr Cameron brushed aside speculation that he was feeling threatened by the prospect of Mr Johnson seeking to return to the House of Commons and challenging him for the leadership of the Conservative Party.

He insisted he had "the opposite of tall poppy syndrome", and wanted to see high-profile Conservatives like Mr Johnson grabbing headlines.

A poll on the first day of the Tory conference in Birmingham suggested that 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.

The poll also showed a strong pick-up in Ed Miliband's approval rating, which has risen to -10 following last week's Labour Party conference - his best showing in an Opinium poll. Deputy Prime Minister Nick Clegg, however, continues to languish on -48.

Asked on BBC1's Andrew Marr Show whether he felt threatened by Mr Johnson, Mr Cameron said: "I think he is a fantastic London Mayor. I think he does an excellent job. I think he is an enormous credit to the Conservative Party.

"I have got the opposite of tall poppy syndrome. 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. Boris is fantastic like that. He is one of those politicians people warm to and I think it is great that we have in our party figures like that."

Foreign Secretary William Hague laughed off suggestions that Mr Johnson's popularity represented a threat to the current Tory leadership.

Mr Hague told Sky News: "He is a great figure in our party, someone we have all campaigned for to be Mayor of London, a great advertisement for our party, achieving a lot in London. We all want to support and share in his success, and I find that is his attitude to the Government. I don't think there is any difficulty about that.

"There is room in the Conservative Party for big figures. It is not the sort of party, and David Cameron is not the sort of leader, who resents somebody else being a big figure. So I think that is all to the good."