Tory backbenchers are threatening rebellion if ­ministers break the pension triple-lock promise as experts warn many ­retirees are sliding deeper into debt.

Under a manifesto pledge the state pension rises by the rate of inflation, average earnings or 2.5% – whichever figure is highest.

But ministers last week held talks on the triple-lock after soaring inflation from a pandemic bounceback meant pensioners would get a rise of more than £700 next year.

The planned betrayal sparked fury on the backbenches.

One Tory MP said: “This is madness. I know they’ll try and sell it by saying the young shouldn’t shoulder the cost of the pandemic, but this is not on.

“Not only does it break a promise we’ve held for a long time, this will alienate our core vote.”

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Breaking the pledge would prove a blow for many retirees after research showed a third of those retiring this year have debts, with £20,650 owed on average.

The report by finance specialist Key revealed the average debt was a fifth higher than that of people who retired last year – although the ­number of those in debt has dipped slightly from 33% in 2020 to 32%.

The proportion with credit card debt fell. But those with overdrafts rose from 10% to 17% and those relying on friends and family up from 8% to 10%.

Key boss Will Hale said: “The amount owed has increased more than £3,000 in 12 months.

“This seems to suggest that those in debt are finding it harder than ever to repay their borrowing.

“Trying to repay debt from a fixed income can be extremely difficult and people are likely to struggle.”