The Conservatives should become the first party to admit the pensions triple lock is becoming unsustainable, according to a senior MP.

The MP said ditching the ‘sacred cow’ pledge, which adds billions to annual pension spending, would demonstrate that the Tories are the party of fiscal sustainability.

Under the radical proposals, retired people would be reassured that their incomes would not be cut – but they would not rise by as much each year because of the ‘perilous’ state of the public finances.

Instead of the current system, which guarantees that the state pension increases in line with whichever is highest of inflation, wage rises or 2.5 per cent, a ‘smooth triple lock’ could be introduced equivalent to the average of the three measures.

Another option would be a ‘double lock’ under which pensions are boosted by the best of inflation or earnings.

The abandonment of the triple lock, a flagship Tory pledge for more than a decade, would have to be accompanied by other measures and messages for older voters who may feel it is unfair.

The senior Tory MP told a Westminster event: ‘It is a simple mathematical calculation that the triple lock is unsustainable in the medium term.’

And they added that the Conservative party needs to ‘slaughter some sacred cows’.

The triple lock currently guarantees that state pensions rise each year in line with whichever is highest of inflation, average wage growth or 2.5 per cent

‘The fiscal situation is increasingly perilous because of what this Government has done and it might mean more emergency surgery than anyone would like,’ the MP said.

The Tories are currently focusing on reducing the amount spent on in-work benefits, with £23billion of savings already identified if they win the next election including restricting welfare payments to British citizens.

But the MP admitted that ‘more may be needed’ including major changes to the triple lock to put it on a ‘more sustainable trajectory’.

They said the party ‘needs to consider long-term sustainability’ while also being fair to people who have paid taxes for decades.

As well as reducing public spending, it is hoped the move would also help cut Government borrowing costs by reassuring bond investors.

It comes just days after a prominent business group called for the triple lock to be scrapped.

The British Chambers of Commerce said that increasing pensions in line with inflation instead would save the Treasury £3.3billion in two years.

And the Institute for Fiscal Studies said that spending on the state pension, which now stands at £154bn, is £16bn a year higher than it would have been without the introduction of the triple lock in 2011.

However although growing numbers of prominent Tories are questioning the commitment, it remains party policy.

Former Work and Pensions Secretary Baroness Coffey told the House of Lords just last week: ‘To some extent, I would actually probably prefer some kind of double lock.’

Ex Chancellor Sir Jeremy Hunt said in April that pensioners ‘might reconsider whether they think it’s such a good policy’ if they knew it was adding to the national debt.

Sir Mel Stride admitted it was ‘unsustainable’ when he was Work and Pensions Secretary but stressed this was only in the long term.

Kemi Badenoch insisted a year ago: ‘The Conservatives created the triple lock; we’ve always protected it and we stand by it.’

But when asked last November if the Tories would never reform the policy, the party leader replied: ‘That moment is not now.’