David Cameron has ruled out a third term as Prime Minister if he remains in 10 Downing Street after the May 7 general election.
Mr Cameron said he will serve for a full second term - which could last until 2020 - if elected, but added: "Terms are like shredded wheat - two are wonderful but three might just be too many."
He named three of his senior colleagues - Home Secretary Theresa May, Chancellor George Osborne and London mayor Boris Johnson - as possible replacements as Conservative leader when he stands down.
Labour accused the PM of "arrogance" and taking voters for granted. But Downing Street sources insisted that Mr Cameron was focused on the need to win the election in six weeks' time.
Although the PM had made clear he would not serve a full third term, no decision had been made on whether he would fight another election or hand over to a successor before the poll.
"We will cross that bridge when we come to it," said a Downing Street source.
"He is clear that we need to win the election in 2015 first. As the Prime Minister has said a number of times, he fully intends to serve a full second term if he wins that election."
In an interview with BBC News, Mr Cameron was directly asked if he would "go for a third term" if he remained PM after the election.
He replied: "No, I think I'm standing for a full second term."
And he added: "I'm not saying all prime ministers necessarily definitely go mad, or even go mad at the same rate, but I feel I've got more to bring to this job, the job is half done, the economy's turned round, the deficit is half down and I want to finish the job."
He added: "There definitely comes a time where a fresh pair of eyes and fresh leadership would be good, and the Conservative Party has got some great people coming up: the Theresa Mays, and the George Osbornes, and the Boris Johnsons. You know, there's plenty of talent there. I'm surrounded by very good people. The third term is not something I'm contemplating."
Mr Cameron said political leaders should never regard themselves as "indispensable".
"Countries, like big organisations, benefit from strong and consistent leadership but there comes a time when you want a fresh pair of eyes and a fresh agenda," he said.
"Certain things that other people would bring, and so you must never think that you're indispensable. However mad you go in this job.
"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."
Labour campaign strategy chairman Douglas Alexander said: "The Tories are taking the British public for granted.
"It is typically arrogant of David Cameron to presume a third Tory term in 2020 before the British public have been given the chance to have their say in this election. In the UK it is for the British people and not the Prime Minister to decide who stays in power."
"Instead of focusing on themselves, it is time we had a Government focused on the needs of working families. Another term of this Government would mean working people worse off and the NHS under threat because of their extreme spending plans. We need a better plan for a better future. We need a Labour government."
Mr Cameron's announcement caused surprise in Westminster, where Tony Blair's 2004 announcement that he would stand down before the end of a third term was widely seem as one of his biggest political mistakes.
The move did not end pressure on Mr Blair to announce the exact date of his departure, and he was eventually forced out earlier than planned. Mr Cameron's announcement is all the more surprising because he has not faced the same pressure as Mr Blair did to hand over to a successor.
His pledge to serve a full second term will raise some eyebrows in Westminster, where some had expected him to be forced to bring a close to his premiership following his planned 2017 referendum on EU membership, whatever the result.
In the latest of a series of BBC profiles of the private sides of party leaders, Mr Cameron was seen cheering on son Elwen's football team from the sidelines, shopping in the local butcher's and preparing food in the kitchen of his Oxfordshire home.
Mr Cameron paid tribute to his wife Samantha for keeping him "sane" in Downing Street, and revealed she will be playing a role in the Conservative election campaign.
"The fact that we do different things helps actually," he said.
"She keeps me sane because she's one of the most organised people that I've ever come across, so home life, the children's life, everything is just brilliantly organised.
"Otherwise you wouldn't get this family time - unless you're really well organised everything would be blown off course, you wouldn't get the chance. She's amazing like that.
"But we'll be out on the campaign trail because we're passionate about this election and what comes next. She is right behind me and what I'm trying to do."
He added: "She will be out there campaigning with me some of the time, she will be out there on her own supporting Conservative candidates some of the time, but she has also got a job and we've got three children."
Mrs Cameron made clear she hopes her husband will remain at Number 10, telling the BBC: "He is definitely in my mind the best man for the job."
She added: "I hope me and the children help him keep things in perspective, keep him grounded, help him pace himself over the next eight weeks."