Cabinet sources have told The Mail on Sunday that Mr Blair sees Environment Secretary David Miliband, not Mr Johnson, as his ideal successor.

'Tony feels that Alan is good but has far too little experience to become Prime Minister,' said one source. 'He says he is a lightweight and too much of an unknown quantity. You cannot aspire to be Prime Minister unless you have been tested - and Alan hasn't.

Instead, the PM is ready to throw his weight behind the even less well known figure of Mr Miliband, who ran Mr Blair's Downing Street policy unit when Labour was elected in 1997.

"David is brilliant, much more personable than he seems, and knows how to revive New Labour. For all his common touch, I don't think Johnson could do that."

"David is bright enough to be Prime Minister but he is too much like a boffin. He is Labour's John Redwood."