DAVID Cameron has admitted he has no desire to emulate Margaret Thatcher and declare that he wants to "go on and on" by unexpectedly ruling out serving a third term as Prime Minister.

Labour swiftly moved to brand the Conservative leader "arrogant" by taking voters for granted given he has yet to win a second term in power while the Liberal Democrats were also critical, labelling his remarks "incredibly presumptuous".

Mr Cameron's move means that, even if he does re-enter Downing Street after the result on May 7, he has opened up a Pandora's Box of speculation about who might succeed him.

In an interview with the BBC, the PM even mentioned three colleagues as potential successors: Chancellor George Osborne, who will be seen as the continuity candidate; Home Secretary Theresa May, highly regarded by right-wing traditionalists, and London Mayor Boris Johnson, who will be viewed as the colourful if somewhat unpredictable dark horse.

Mr Cameron made clear he would serve for a full second term - which could last until 2020 - if elected but then added: "Terms are like shredded wheat; two are wonderful but three might just be too many."

Yet, given his remarks, doubts have now arisen if he would indeed serve a full five-year second term. A Downing Street source admitted no decision had been made on whether he would fight a third election or hand over to a successor before the 2020 poll.

"We will cross that bridge when we come to it," he explained.

This leaves open the option, that if he won the May election, held a poll on Britain's membership of the EU in 2017 and won that, Mr Cameron could then decide to stand down, having successfully negotiated two referendums; one on Scotland staying within the United Kingdom and one on the UK staying within the European Union.

Mr Cameron said the "job is half done, the economy's turned round, the deficit is half down and I want to finish the job," the PM 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, the George Osbornes and the Boris Johnsons.

"There's plenty of talent there; I'm surrounded by very good people. The third term is not something I'm contemplating."