Vulnerable households across the UK are set to fork out an extra £400 million to UK energy firms after failing to gain assistance through an extended price cap, a new report shows.
Ofgem data compiled by energy provider Bulb found that while the energy watchdog's prepayment price cap was extended to around one million vulnerable customers this winter, a further 3.9 million customers who need assistance will be stuck with pricey bills.
Ofgem has said that the cap is expected to save protected households around £120 per year on average.
It means that 3.9 million households - which includes customers on income-based or disability-based Government benefits - are missing out on potential combined savings of around £429 million, Bulb said.
Bulb said it was appealing to the Big Six energy firms - comprised of British Gas, SSE, E.ON, Npower, EDF and Scottish Power - to do "more to help those who Ofgem recognise are in desperate need", as a majority of those vulnerable customers are estimated to be under their tariffs.
Bulb co-founder Hayden Wood said: "We're pleased to see Ofgem doing everything they can to safeguard vulnerable households, but we believe energy suppliers also have a responsibility to stop struggling customers being overcharged on their bills as the months get colder.
"Families struggling to heat their homes in winter is a tragedy, and something that has no place in 21st century Britain," Mr Wood added.
"We believe it is time for those companies to step up and end the injustice of those least able to pay being charged the highest rates," he said.
Bulb said the Big Six should reduce their Standard Variable Tariff (SVT) rates over the colder winter months.
Britain's biggest energy producers are currently bracing for a raft of regulatory changes after the Government announced last month that a price cap will be imposed on poor-value energy tariffs.
The plans have drawn criticism from the Big Six, who are soon set to become the Big Five after SSE and Npower confirmed that the have agreed to merge their operations, creating a new listed energy supplier in the UK.