British Prime Minister David Cameron has ruled out seeking a third term in office if he wins a second mandate in a tightly fought national election on May 7, saying it would then be time for a new leader of his right-leaning Conservatives.

His comments are designed in part to underscore his party's message that it believes its job of rebuilding the economy and reforming Britain is only half-done and that it needs another five years "to finish the job".

Cameron, whose party is neck-and-neck in opinion polls with the opposition Labour Party, also said he would like to serve a full five-year second term from 2015-2020 if re-elected, scotching persistent speculation that he would stand down after a possible European Union membership referendum in 2017.

But he suggested his political ambitions would then have run their course.

"I'm putting myself forward for the full five years. I feel fit enough and healthy enough to do the job. I've got a real passion for it and I'm really keen to win. If I fall short I'll be disappointed," he told BBC TV.

But asked whether he wanted a third term, he said: "No ... The third term is not something I'm contemplating."

Cameron, 48, said possible successors included Theresa May, the interior minister, London mayor Boris Johnson, and finance minister George Osborne, saying the time would then come for "a fresh pair of eyes and fresh leadership".

If Cameron does win re-election, his comments are likely to spark a lengthy leadership contest to succeed him.

Cameron became prime minister in 2010 and has headed Britain's first coalition government since World War Two, with the centre-left Liberal Democrats as the junior partner to his Conservatives.