David Cameron was forced to defend his decision to announce a sell-by date on his premiership yesterday as senior Tories accused him of trying to anoint George Osborne as his successor. Downing Street was struggling to contain the fallout from the Prime Minister’s extraordinary announcement that May’s general election will be his last. Friends dismissed rumours that Mr Cameron’s wife, Samantha, had asked him to make clear he would not seek a third term if re-elected, while Mr Cameron insisted he had simply given a ‘straight answer to a straight question’ and that most people would think that was a ‘sensible’ thing to do. Scroll down for video . Prime Minister David Cameron (pictured) was forced to defend his decision to announce a sell-by date on his premiership yesterday as senior Tories accused him of trying to anoint George Osborne as his successor . As Labour accused him of ‘arrogance’ for discussing a third election before he wins a second, he insisted he was ‘taking absolutely nothing for granted’. ‘My entire focus is on the next 44 days and the general election that will decide which team runs the country for the next five years,’ Mr Cameron said. But senior MPs warned that pre-announcing a departure before the 2020 election would inevitably undermine his authority if he remained in No 10. It came as Mr Cameron was reminded of his comments to Tony Blair when the former premier announced he was handing over the reins to then-Chancellor Gordon Brown mid-term. ‘Your authority is draining away,’ he told Mr Blair in the Commons in January 2007. ‘The Government can’t plan. Ministers are treading water. They are all waiting for the Chancellor and not listening to you.’ One former shadow minister said that by indicating he will depart in the next Parliament, Mr Cameron appeared to be seeking to head off moves to oust him on May 8 if he falls short of a Commons majority and tries to lead another coalition or a minority government. David Cameron has revealed he will stand down as Prime Minister before 2020 if he wins the next election . Senior MPs warned that pre-announcing a departure before the 2020 election would inevitably undermine his authority if he remained in No 10. Pictured: David Cameron visits a butchers in his Oxfordshire constituency . ‘He may think it will be harder for the men in the grey suits to come knocking when he has said he won’t be going on and on,’ the MP said. ‘The truth is he won’t last five minutes, let alone five years, if he loses again.’ There was particular consternation that Mr Cameron effectively fired the starting gun on a leadership contest by naming three potential successors: Home Secretary Theresa May, London Mayor Boris Johnson and Mr Osborne, the Chancellor. Mrs May and Mr Johnson have long been seen the leading rivals to be the next Tory leader, but Mr Cameron sparked frenzied speculation at Westminster by including his name of Mr Osborne, his closest ally. Since his unpopular Budget of 2012, Mr Osborne’s leadership prospects have been discounted by many. Boris Johnson . Boris Johnson is the favourite among voters to succeed David Cameron, a poll reveals today. But the London Mayor repeatedly refused to say whether he would join a leadership contest. Chancellor George Osborne is thought to be the Prime Minister’s preference to be his successor. But in a ComRes survey for the Daily Mail, Mr Johnson was backed by 31 per cent of voters, ahead of Theresa May on 17 per cent and Mr Osborne on 12 per cent. Support for the Mayor among Tory voters was particularly strong – 36 per cent said he would make the best prime minister, Mr Osborne was on 25 per cent and Mrs May on 18 per cent. Mr Johnson’s strongest backing was among Ukip supporters, at 44 per cent – suggesting he would do the best job of regaining support from the Eurosceptic party. He was also backed by 32 per cent of Labour voters, ahead of Mrs May on 19 per cent, and the Chancellor on 7 per cent. But in an LBC Radio interview yesterday, Mr Johnson dodged questions about his ambitions more than a dozen times. He is bidding to return as an MP in May in the safe seat of Uxbridge, but claimed Mr Cameron’s intervention was ‘no big deal’. ‘All he’s saying is I think obvious and common sense … he doesn’t want to go on and on and on like Mrs Thatcher,’ the Mayor said. ‘To say he’s arrogant or presumptuous to rule [out] a third term seems to be totally bizarre because actually he’s being rather humble and rather straightforward and telling it like it is.’ Asked repeatedly if he would go for the party leadership, Mr Johnson said: ‘I can’t possibly answer that because it’s a long way off. We probably don’t even know the identity of the next leader … It is some baby, unborn.’ Mr Osborne praised Mr Cameron for giving ‘a direct answer’. Mrs May is yet to comment. But the recent turnaround in his popularity as the economy improves has led many MPs to reconsider. Polls suggest Mr Osborne has become the most popular Conservative Treasury spokesman since Nigel Lawson in 1987. ‘Cameron has propelled George Osborne into the top three, quite deliberately,’ said one senior MP. ‘Whether that helps George or not remains to be seen.’ Another former minister said: ‘It’s not what he has said about timing – it’s the list of names. The Parliamentary party doesn’t really want any of them. Theresa May is a cold loner with a following of about two. George has been a good Chancellor of the Exchequer, but people resent his coterie of apparatchiks and think he is ultimately unelectable. Boris is another Bullingdon Club boy who ignored Parliament when he was last in it and lots of people think would be too risky.’ MPs also questioned how Mr Cameron could promise to serve a ‘full second term’ when a new leader would need at least six months to establish themselves before the 2020 election. Some allies of the Prime Minister, however, suggest he could step down as party leader, allowing a contest to take place some months before, while remaining Prime Minister right up until polling day. While this would be unprecedented in modern British political history, it would mirror the American system, where a president serves a fixed second term. Former Tory minister Michael Portillo described Mr Cameron’s announcement as ‘bizarre’. He told the BBC: ‘When you play chess you have to consider the next two moves, and I don’t think he did. There is no point setting off on this wild goose chase in the middle of an election campaign.’ But Defence Secretary Michael Fallon defended the Prime Minister. ‘He made very clear he is standing for a full second term. What happens in 2020 really, I think, doesn’t bother a lot of people,’ he said. ‘If he had said exactly the opposite – that he wanted to go on and on and he wasn’t prepared to answer the question when he was likely to finish – you’d have accused him, quite rightly, of arrogance and wanting term after term.’ A snap poll by ComRes for the Daily Mail found voters are divided over Mr Cameron’s announcement. Some 70 per cent believe Mr Cameron was ‘just being honest’, while 80 per cent say ten years is long enough for any Prime Minister to achieve their plans. Four in ten said he had made himself a lame duck, but the majority – 59 per cent – disagreed. However, 70 per cent said he should be focusing on voters rather than his own job. Four in ten think the next government would be destabilised by a Tory leadership election.
David Cameron said earlier this week that he would not bid for a third term . Cameron said two terms were 'wonderful' but three 'might just be too many'
Named Boris Johnson, Theresa May and George Osborne as successors . Has now insisted he was just giving ‘straight answer to a straight question’
PM also said he believed most people would think it was a ‘sensible’ thing . Mr Cameron said yesterday Thomas the Tank Engine was the television character he most identified with. In an interview with Netmums.com, Mr Cameron said: ‘He’s a British icon, he’s blue, he’s loyal and reliable – and he always saves the day.’ Lib Dems leader Nick Clegg opted for Paddington Bear, while Labour’s Ed Miliband sees himself as Captain Barnacles.