Pre-payment energy meter customers are missing out on average savings of £100 a year by not switching supplier, the regulator Ofgem has said.
Four million households spend more than 10% of their income on fuel
This is a much bigger saving than direct debit customers could save by switching, the regulator added.
Consumer groups have long criticised the pre-payment energy market as being expensive, with suppliers making it difficult to switch.
Generally, pre-payment customers tend to be on low incomes.
Therefore, pre-payment customers should look to make savings by switching supplier if they already have not done so, Ofgem stressed.
"Pre-payment customers should take advantage of the competitive market if they are to see further falls in their energy bills," Alistair Buchanan, Ofgem chief executive, said.
"By changing supplier they can save on average around £100. But in some regions this can be as great as £170 per year," he added.
In the UK, four million households spend more than 10% of their income on energy and are therefore defined as fuel poor.
Many of these households use pre-payment meters.