Chancellor Rachel Reeves has defended her decision to scrap winter fuel payments for around 10 million pensioners.
She told the BBC she had found a “black hole” in the public finances and “had to act” to “fix the mess”.
But a former pensions minister said she was “shocked” by the decision to restrict the fuel payments.
Ms Reeves has accused the previous government and former chancellor Jeremy Hunt of hiding a massive shortfall in public money, which Mr Hunt has strongly denied.
Ms Reeves said she had been forced to make “tough decisions” after the government said it had uncovered a £22bn hole in the public finances.
One of the decisions she announced on Monday was that pensioners in England and Wales not on pension credit or other means-tested benefits will no longer get winter fuel payments worth between £100 and £300.
Former Conservative pensions minister Baroness Ros Altmann told the BBC she was “shocked that the chancellor has chosen to take money away from some of the poorest people in this country”.
Roughly 850,000 households who are eligible to receive pension credit do not claim it, according to figures released by the Department for Work and Pensions last year.
Baroness Altmann said many do not claim because they are “too proud” to do so.
Ms Reeves said pension credit would be merged with housing benefit so more people who are entitled to it will claim, and the government would work with older people’s charities and local government to increase take-up.
But the move was also criticised by charities including Age UK, which said on Monday that “as many as two million pensioners who badly need the money to stay warm this winter will not receive it and will be in trouble as a result”.
“At the other end of the spectrum well-off older people will scarcely notice the difference – a social injustice,” said charity director Caroline Abrahams.