David Cameron today said he wanted his potential rival Boris Johnson “to be as ambitious as possible”.
The London Mayor has been tipped to succeed Mr Cameron as Tory leader, despite admitting voters would be unlikely to elect "a prat" like him.
But speculation has continued to link Mr Johnson with a challenge to the Prime Minister, as the Mayor's profile has soared during the Olympics.
Mr Cameron said today: "I want Boris to be as ambitious as possible. He is a fantastic London Mayor."
The Prime Minister claimed he wanted to see city leaders elevated into national politics and denied he wanted to "put a cap on people's ambitions".
He told ITV1's Daybreak: "It's good for my party's point of view that we have got big figures that can do big things for our country, so I welcome the fact I have got in Boris Johnson a great Mayor of London, a great titan in my party and someone who I think has got a lot more to give the country."