David Cameron will not serve a full second term as Prime Minister if he wins the election, Iain Duncan Smith admitted this morning.

The Work and Pensions Secretary became the first Conservative Cabinet minister to accept there would have to be a leadership contest before the 2020 election.

It is a significant departure from the party’s official line and Mr Cameron’s insistence he would serve a full second term.

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The Prime Minister ruled out serving a third term in a BBC interview last week but insisted he would serve a full second term in a move to dampen speculation that he would step down after winning a referendum on the EU in 2017.

But asked on the Andrew Marr Show whether Mr Cameron would have to stand aside for a leadership contest in the Conservative party before the 2020 election, Mr Duncan Smith said: “Yes, that’s right, he does... Of course there will be a competition at some point, but that will be a competition on the back of a successful Prime Minister doing something that most Prime Ministers have never done before: saying ‘I know when it's time to go’.

“You’ve have to literally wrap the knuckles off people like Gordon Brown and previous Prime Ministers to get them to think about it, but we’re actually very keen to say there is a limited, there’s an amount of time a Prime Minister should serve before they get stale and he’s right about that.

“We are in a different world now than we were when I was leader or when William Hague was leader. In those days we did not have fixed term parliaments, the Prime Minister called an election after three and a half years or four years if they wanted, so it was very difficult to say what a full term was. Now, under a fixed term Parliament you know it is five years.”

Mr Smith said he would be 'sorry' to see David Cameron go (Reuters)

He added: “I’ll be sorry to see him go, as and when he chooses to do that.”

The Work and Pensions Secretary was also grilled about his party’s pledge to find £12 billion of savings from cutting the welfare budget but he refused to give any more details about where they would come from.

Around a quarter of the £12 billion of cuts have been laid out by the Tories, such as lowering the cap on household benefit claims from £26,000 to £23,000 and a further freeze on working-age benefits.

But they may not reveal any more details of how they plan to find the savings before the election, Mr Duncan Smith said.

This has invited accusations of dishonesty but Labour has also so far refused to lay out the vast bulk of how they intend to make savings.

A leaked document from the Department for Work and Pensions appeared to suggest the party were planning changes to industrial injuries compensation, child benefit, the carer’s allowance and disability benefits, but Mr Duncan Smith insisted it did not represent government or Conservative party policy and blamed the leak on a “bitter individual” in the department.

Tories are planning more cuts to disability benefits, a leaked document suggests (Getty Images)

He insisted his party would not make “cheese-paring cuts” but added that it may not be “relevant” to explain where the rest of the cuts will fall before the election.

“When we are right and we are ready, we will talk about what we plan to do,” he said.

“[Voters] know for certain that we are going to save the £12 billion. We may, we may not, decide that it’s relevant to put something out there about some of those changes.

“No decisions have been made, as and when decisions have been made of course we will be very open to the public.

“I haven’t made decisions and we haven’t made decisions, it would be dishonest to say we’ve made decisions but we’re just going to keep it all secret, that’s not the case.

He added: “What I will say is there are things we want to do that are of life-changing dramatic effect and that is about getting people back to work and improving their life chances.”

Rachel Reeves MP, Labour’s Shadow Work and Pensions Secretary, responding to Iain Duncan Smith’s interview on The Andrew Marr Show, said:

"The public have a right to know who will be hit by the Tories' plan and they must now come clean on their £12 billion cuts.

"Iain Duncan Smith's refusal to admit how children, disabled people, carers and working families will be hit by secret Tory plans six weeks before the election is completely unacceptable.