The Liberal Democrats have called for an extension to Saturday's deadline to access winter fuel payments, warning tens of thousands of older people could miss out.
There has been a surge in applications for pension credit since Chancellor Rachel Reeves announced in July that the payment for this year would be mainly limited to those receiving the benefit.
But as of mid-November, external, thousands more pensioners believed to be eligible had not yet applied.
At this point there was also a backlog of 91,000 unprocessed claims, with around 9,000 being processed each week, according to the latest official figures, external.
The government says it has been working to boost take-up and eligible claims can be backdated.
People have until the end of Saturday to apply for pension credit to get this year's winter fuel payment.
The deadline for online applications is 23:59 GMT, while the Department for Work and Pensions (DWP) said its claim phone lines would also be open from 08:00 to 15:15.
The Lib Dems are calling for this deadline to be extended to 28 February and for applications to be processed quicker, to ensure vulnerable pensioners do not miss out.
Previously all pensioners got the payment to help with energy costs, worth between £200 and £300.
But in July the government said future payments in England and Wales would only go to those on low incomes who received specified benefits such as pension credit, meaning more than 10 million people will no longer be eligible.
Prime Minister Sir Keir Starmer has defended the move, saying tough decisions were necessary because of the state of the public finances.
But it has been criticised by several charities, unions and opposition parties, with some Labour MPs also raising concerns about the impact on low-income pensioners.
Lib Dem leader Sir Ed Davey said: "Older people across the country are having to make heart-breaking decisions over whether to heat their homes or eat over the Christmas period.
"Ministers need to do everything they can to limit the damage of their wrong-headed decision to slash Winter Fuel Payments for millions of pensioners."
Following its announcement, the government launched a campaign encouraging those eligible to claim pension credit, with around 150,000 new applications between 29 July and 17 November.
But this represents a small fraction of the 880,000 pensioners estimated to be eligible who had not put in a claim.
Age UK says it has been inundated with requests for help to apply for the benefit.
The charity's director Caroline Abrahams told the BBC: "For some older people, they still haven't heard about pension credit or the form is too overwhelming.
"There is help available but of course the system is overwhelmed at the moment."
Analysis by BBC Verify suggests tens of thousands of pensioners are unlikely to receive their Winter Fuel Payment before the end of the year because of the backlog in unprocessed claims.
The DWP says it has deployed 500 additional staff to speed up processing.
A government spokesperson said: "Over one million pensioners will still receive the winter fuel payment, and our drive to boost pension credit take-up has seen applications more than double with over 40,000 more pensioners now receiving it, as well as the winter fuel payment.
"We continue to urge anyone who thinks they may be entitled to pension credit to check now, as all eligible claims can be backdated, and anyone who makes a successful claim before December 21 will receive their payment."