The Prime Minister refused to say if the number would be higher or lower than the 4,000 deaths that Labour previously warned means-testing the payments would lead to.
Sir Keir insisted the triple-lock means pension increases "will outstrip any loss of payment". And he also refused to rule out axing free bus passes and the single-person council tax discount.
During a clash in the Commons Conservative leader Rishi Sunak said: "We know why he's hiding the impact assessment.
"Labour's own previous analysis claimed this policy could cause 3,850 deaths.
"So, are the numbers in his impact assessment higher or lower than that?"
Sir Keir insisted the decision had to be made to fill a £22billion blackhole in the public finances left by the Tories.
He added: "We're taking this decision to stabilise the economy.
"By committing to the triple lock we can make sure that payments of state pension are higher and therefore there's more money in the pockets of pensioners, not withstanding the tough action we need to take." Mr Sunak hit back: "Today pensioners watching will have seen the Prime Minister has repeatedly refused to admit or publish the consequences of his decision and we will continue holding him to account for that."
Labour later denied it had broken its promise to be transparent by refusing to publish the impact assessment.
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