Many households with traditional prepayment meters have not yet claimed their £400 voucher to help with energy bills, BBC News has reported.
Customers should have begun to receive the first £400 instalment of the government's Energy Bill Support Scheme on 1 October.
Payment company PayPoint told BBC News it had expected to process 800,000 vouchers in October, worth a total of £52.8mln, but so far only £27mln has been redeemed. The vouchers are valid for just 90 days.
The government urged consumers to use the credit as soon as possible.
Under the government’s scheme, every household's energy bill will be cut by £400. This will be applied over six months, with a reduction of £66 in October and November, and £67 every month between December and March 2023.
Customers with a traditional prepayment meter, which requires topping up via a key or card in a local newsagent or Post Office, will get the support in the form of a voucher.
BBC News said PayPoint and fuel poverty charities are urging any prepayment customers to go through their post in case they have missed the delivery.
More than four million people in the UK have a prepayment meter, and around half of those are the older, non-smart type of meter, according to the energy watchdog Ofgem.
"It's so important that everyone gets the support they're entitled to," Gillian Cooper, head of energy policy at Citizens Advice, told BBC News.
Citizens Advice said that by the end of September this year, it had seen more people unable to top up their prepayment meter than in the last three years combined.
A spokesperson for the government’s business department said: “We encourage customers to have their credit applied to their meter as soon as possible so that they can benefit from the scheme, which comes on top of the energy price guarantee which is saving the typical household about £700 this winter.”