Currently the government estimates 1.9 million pensioners – around 15% – are in relative poverty.
The new estimates, published on Tuesday, suggest the cuts to the winter fuel payment would increase pensioner poverty by 0.5 percentage points.
A person is considered to be living in relative poverty if they have less than 60% of the median income.
In her letter, Kendall said the work and pensions department had written to 120,000 pensioners to encourage them to claim the pension credit to which they may be entitled.
She added that the decision to cut the winter fuel payments was “not a decision this government wanted or expected to take”.
“However, we were forced to take difficult decisions to balance the books in light of the £22bn black hole we inherited.
“Given the dire state of the public finances, it’s right that we target support to those who need it most.”
Helen Whately, the Conservatives’ shadow work and pensions secretary, said: “Finally the dam breaks and we get to see what Labour have known all along.
“Their winter fuel payment cuts are going to plunge 100,000 pensioners into poverty in the next few years.
“Clearly Keir Starmer feels like that’s a price worth paying to make a political point. But I don’t think those pensioners would agree with him.”
Daisy Cooper, the Liberal Democrats’ Treasury spokesperson, said: “While the Conservatives undoubtedly left this government a dire fiscal inheritance, that’s no excuse to push more pensioners into poverty as the temperature plummets.”
During a press conference in Brazil, where the prime minister is currently attending a G20 summit, Sir Keir Starmer was asked about the figures.
He said that the state pension would rise by £470 in the spring and that pensioners would be better off.
Earlier in the day, Scottish Labour leader Anas Sarwar diverged from Sir Keir by pledging to increase eligibility for pension credit if they formed the next government.
