Boris Johnson as Tory leader 'would wipe out Labour lead' Conservative supporters preferred Mr Cameron by a margin of 58 per cent to 29 per cent. However, Ukip supporters opted for Mr Johnson by 51 per cent to 21 per cent. The poll was carried out in the days after Mr Johnson floundered as he was questioned on the BBC's Andrew Marr Show over episodes from his past. Eddie Mair, the interviewer, quizzed him about an alleged extra-marital affair, fabricating quotes as a journalist and agreeing to help a friend with a plot to get someone beaten up. The London Mayor has dismissed complaints about his treatment, saying it was the BBC's job to "have a bash" at him. Voters also appear to think the questions were justified, with 55 per cent saying the questioning was acceptable and 23 per cent saying it was not. However, the episode has not negatively affected his popularity. The poll will add to pressure on Mr Cameron who has been the subject of speculation about his leadership after the Conservatives suffered a bruising defeat to both the Liberal Democrats and Ukip in the Eastleigh by-election. It will also add to Mr Johnson's reputation for brushing off embarrassing incidents. The London Mayor was interviewed on the BBC ahead of a documentary about his life by political film-maker Michael Cockerell. In the hour-long piece, Mr Johnson admitted for the first time that he would "of course" like to take over from David Cameron as leader of the Conservatives. In a series of interviews, many of his friends, family and colleagues portrayed the London Mayor as a canny operator with deep ambition beneath his bumbling exterior. His sister, the journalist Rachel Johnson, said the London Mayor "sees life as a competition and he always wants to be top." Mr Johnson is not even an MP anymore and has three years left as Mayor of London. However, he could return to Parliament after the next election if a Conservative MP were willing to give up their seat for him, triggering a by-election.