A former Conservative leader has called on Theresa May to scrap the ‘triple lock’ on the state pension because of rising costs.

Iain Duncan Smith, who quit as Work and Pensions Secretary in David Cameron’s cabinet earlier this year in protest at another raid on the social security budget, said that the money would be better used to take the "pressure off" those still working.

He also warned that the policy was in danger of creating friction between the generations.

Former chancellor George Osborne came under fire in the run up to last year's General Election from critics of his pledge to cut £12billion from the welfare bill.

But Mr Duncan Smith said that the 'triple lock', which guarantees that the state pension will rise by the higher of average earnings, inflation or 2.5 per cent, had cost £18bn this year.

He told the BBC’s Sunday Politics programme that in 2010: “When we came in (to government) we had a commitment as a Conservative Party in our manifesto to get pensions back onto earnings. That was absolutely right. With the Coalition that was moved to a triple lock which guaranteed a minimum of two and a half per cent.”

He went on: “I’m in favour now of getting this back to earnings again. And allowing it to rise at a reasonable level.”

He said that in his previous role he had been "under pressure... scratching around trying to take more money out of working age areas, when in actual fact the budget was almost out of control on the pensions side. I’m in favour of helping pensioners but I think that now that they’re up to a reasonable level, at a steady rate that can be afforded by government that takes the pressure off working age people who have to pay for that.”

Scrapping the lock would also “help with the inter-generational fairness argument, which I feel is very strong”.

A senior adviser to Mrs May has previously suggested that the lock should be cut to avoid hitting working families.

Nick Timothy, the Prime Minister's joint chief of staff, said that tackling the triple lock was the "obvious alternative".

Separately, former pensions minister Baroness Altmann has suggested instead a "double lock" instead, where the state pension increases by the higher of the rise in average earnings and inflation.

In August No 10 rejected calls to be scrap the lock, suggesting that it would remain in place until at least 2020.

At the Conservative party conference in Birmingham earlier this month, Damian Green, the Work and Pensions Secretary, said his party had a duty to look after the 13m people who receive the state pension.

“That means protecting pensioner benefits and uprating the state pension by the triple lock, because our parents and grandparents deserve to have a secure retirement,” he said.

Mr Duncan Smith also reiterated his call for the cuts in universal credit to be reversed.

The policy is designed to allow workers on benefits to keep more of the money they earn.

But Mr Osborne slashed the rates, as part of his bid to bring down the welfare bill.

Mr Duncan Smith said the cuts should be reversed by delaying plans to raise the point at which people start paying tax to £12,500 a year.

“I don’t believe you need to go through with that continuing raise of the tax threshold. That alone costs – of course it’s dependent on inflation – but give or take £4bn pounds every time.”