David Cameron has hit back at claims he made a strategic blunder by telling the BBC he would not serve a third term as prime minister if he is re-elected.
The Tory leader said he had given a "straight answer to a straight question" and most people would think that was a "sensible" thing to do.
Labour said Mr Cameron was being "arrogant and presumptuous".
But most pundits said Mr Cameron's biggest mistake was to name his possible successors.
The PM suggested Theresa May, George Osborne or Boris Johnson were ready to fill his shoes when he stands down, potentially firing the starting gun on a Tory leadership race.
'No big deal'
Asked what he was thinking of when he made the comments to the BBC's James Landale, Mr Cameron said: "What I did in my kitchen was give a very straight answer to a very straight question.
"And I think people will understand that a full second term, a full five years, is a very reasonable, sensible thing to say."
He said he was "taking absolutely nothing for granted" when it came to May's general election and his "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".
Speaking at an Age UK event in Birmingham at which he faced heckles about the NHS from some of the audience, he said ruling out a third term would prevent "endless political process five years hence" if he returns to power in May.
Former Conservative Cabinet minister Michael Portillo described Mr Cameron's announcement as "bizarre".
He told BBC Radio 5 Live: "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."
Timetable to 2020 - if Cameron wins general election
May 2015: General election. If David Cameron wins an outright majority he will serve a full five-year term
May 2016: Scottish Parliament and Welsh Assembly elections
2017: EU referendum. The outcome will decide Britain's future relationship with Europe. Mr Cameron plans to campaign to keep the UK in the EU but if he is on the losing side he may be forced to step down as Tory leader earlier than planned
May 2019: European elections. If he is still Conservative leader, David Cameron may wish to wait until after the European elections to step down, which would still give his successor time to bed in before the next general election
May 2020: General election
Two of the possible contenders named by Mr Cameron to replace him have given their reaction to his comments.
London Mayor Boris Johnson, who is bidding to return to the Commons on 7 May, in the safe Tory seat of Uxbridge, said it was "no big deal".
"All he's saying is I think obvious and common sense, which is that he doesn't want to go on and on and on like Mrs Thatcher.
"But he's got five more years to complete the work of getting Britain's economic recovery on track, and there's still a lot to do, and he needs to make sure he's there to do it."
Chancellor George Osborne said: "I think it's really refreshing we have a prime minister who gives a direct answer to a direct question,"
Home Secretary Theresa May has yet to comment.
Deputy Prime Minister Nick Clegg joked that he would "happily settle for two terms as prime minister" as he left the coalition government's final cabinet meeting before the election.
The original comments from Mr Cameron came while being interviewed in his kitchen by the BBC's deputy political editor James Landale, as part of series of profiles of party leaders.
He said "I've said I'll stand for a full second term. But I think after that it will be time for new leadership. Terms are like Shredded Wheat - two are wonderful but three might just be too many."
By James Landale, deputy political editor, BBC News
By answering my question, Mr Cameron has potentially opened a Pandora's box.
He has invited Westminster and the country to contemplate a time when he is no longer prime minister and that is a dangerous gamble to make so close to an election.
He is telling the voters that they can back him one last time in May and then they can see the back of him in five years' time.
But his opponents will say he is making an arrogant presumption about the election result, an assumption that he will have the choice over whether to serve a third term.
In a few weeks' time, the voters may make that decision a little earlier for him.
In response, a Labour source said: "It is arrogant and presumptuous of David Cameron to speculate about a third Tory term in 2020 before the British public have been given the chance to deliver a verdict on his first. The Tories are taking the British public for granted."
The Lib Dems said Mr Cameron was being "incredibly presumptuous". A UKIP spokesman said: "Mr Cameron's announcement will create the long-awaited civil war in the Conservative Party over Europe."
As the issue dominated the political day, Mr Cameron was given a tough time from an audience of pensioners when asked what he would do about doctors and nurses leaving the NHS and going to work for agencies.
His answer was met with cries of "you're not answering the question", "rubbish" and "answer the question" from those in the Age UK audience.
After taking questions from the media, Mr Cameron thanked the delegates for their "lively questions and the lively interactions".