The PM is expected to hike retirement pots by 8.5 per cent, in line with the growth in wages.

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Insiders say the pledge will prevent a “politically suicidal” backlash from traditional Tory-supporting OAPs before next year’s election, after fears the lock could be watered down.

But Downing Street played down speculation Mr Sunak could slash death duties in a bid to woo shire Conservatives — despite a clamour from MPs.

Cabinet Minister Grant Shapps branded inheritance tax “deeply unfair”.

But he nevertheless insisted Chancellor Jeremy Hunt was in a “fiscal straitjacket” with little room for giveaways.

He told the BBC: “I just lost a parent and I can understand why people find inheritance tax particularly punitive.

“However, there are lots of different tax considerations for the Chancellor.”

Inheritance tax is levied at 40 per cent but less than four per cent of deaths meet the threshold for the charge — with sources last night saying it would not be scrapped.

Ex-Cabinet Minister Sir Simon Clarke said inheritance tax should not be the priority when it comes to tax cuts.

He said: “If we are choosing, income tax should trump inheritance tax every time.”

The Chancellor will unveil an Autumn Statement this November, where he has all but ruled out tax cuts.

But he is widely expected to dangle giveaways in the Spring Budget — likely the last fiscal event before the election.