Nearly three million households could be in debt before the winter. Two out of five households fear that they will be able to pay their bills.
Despite the hottest summer ever recorded, the number of households owing money has risen by 12 percent since last fall, with the rise attributed to "unprecedented" price increases.
Energy debt has risen by £ 75 million since 2017, and households now owe utilities £ 400 million, according to the comparison website uSwitch.
After a year of price increases, households owe their utilities £ 75m more than they did in the fall of 2017
The study follows 55 price increases from 32 suppliers since January and increases the energy costs of around 12 million standard rate customers by nearly £ 900 million a year.
Britain's largest utility, British Gas, has raised its prices twice this year, causing a "sharp" increase in wholesale energy costs.
This meant that 3.5 million customers would have to pay £ 104 more with their variable fuel-price plan from the beginning of October, when the price increase took effect before the end of May.
At a time when households would typically expect credit before the winter period, 11 percent of consumers said that they actually ended the summer with debt, as their utility paid an average of £ 134 apiece.
41% of customers say they are worried about paying their energy costs in winter, and nearly 3 million households are already at fault with their suppliers
Rik Smith, the company's energy expert, said, "With winter just around the corner, it's important for households to use that time to handle rising bills.
"After so many price increases this year, many people may have received a notice about the price increase in the summer, but have not switched to a cheaper deal."
He added that households now have time to take action.
The survey shows that 21 percent of respondents either hoped that the amount would naturally decline over time or did not know how to pay off the debt.
Three percent said they did not believe they could afford to repay their debts.
uSwitch advises anyone worried about their debts to talk to their energy supplier, who can help with the preparation of a repayment plan, provide energy-saving advice, and sometimes provide free isolation or grants to keep costs down.
British industrial group Energy UK said: "When customers have difficulties, it's most important to get in touch with their utility as soon as possible so they can provide help and support."
71 festive fares have either already ended or will end next month, which may result in customers automatically switching to more expensive standard variable rates if they do not switch.
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