Until at least 2020 – Crabb

of the triple-lock, which ensures pensions rise by 2.5% or – depending on which is highest – inflation or the increase in average earnings.

On the BBC’s Westminster Hour, Mr Crabb said: “I still think we should keep it until 2020. That was the promise we made two years ago.

“I think it would be bad faith to even hint at removing the triple lock any time before 2020 but as we’ve talked about before there are substantial costs to always committing ourselves to increase the basic state pension above inflation and wages in perpetuity.”

He added: “[We] need to be looking at the overall fairness of the welfare budget and whether we’re asking too much in term of pressure on benefits for working-age families.”

Mr Crabb also defended the Conservatives’ goal of reducing net migration levels down to the tens of thousands but acknowledged it would be “extremely difficult if not impossible”.

He said: “Having a target like that is really important for being clear with the electorate about where our direction is, what we want to achieve and being ambitious... My own concern around that, and I’ve said this previously, is that we’ve known for quite some time reaching that target with immigration is extremely difficult if not impossible.”

The Liberal Democrats will have a commitment to keep the triple-lock in their manifesto.

Brecon and Radnorshire candidate James Gibson-Watt said: “Liberal Democrats believe that we should all be able to live with dignity and respect, and that nobody should be left unable to meet their basic needs. Unlike the Conservatives, we are clear that we will protect the triple lock.

Ukip has committed to keep the triple lock until 2020.

A Plaid Cymru spokesman said: “Plaid Cymru will guarantee the triple lock on pensions.”