(Image: TANYA LEAF/Getty)
More than nine million households face winter with no energy credit built up or already behind on payments.
Research by Uswitch revealed two in five bill-payers are more worried about heating costs this year than last.
Households usually build a credit cushion over warmer months through regular direct debits to soften cold-weather bills.
Only 55 per cent of account holders are currently in credit, down from nearly two-thirds in 2022.
Even among those with positive balances, just 29 per cent have stored £300 or more, with the typical customer holding between £51 and £100.
Around 3.4 million households are currently in debt to their energy supplier, owing an estimated combined £750m.
That leaves those in arrears owing an average of £220 each heading into the coldest months.
Just over a quarter of indebted customers owe more than they did last year, while nearly one in 10 have moved from credit into arrears.
One in 10 say they cannot afford to clear what they owe, while 20 per cent hope the balance reduces on its own.
The findings show 40 per cent of households are more anxious about energy bills, up from 28 per cent in 2025.
Anxiety is particularly pronounced among women, with 47 per cent expressing increased worry compared with 33 per cent of men.
Younger adults have been hit hardest by the absence of credit buffers.
One in six people aged 18 to 34 owe money to their supplier, more than double the seven per cent rate seen among over-55s.
Fewer than half of 18 to 34-year-olds hold any credit balance, compared with 64 per cent of older residents.
One in five young adults also rely on prepayment meters, compared with one in 25 over-55s.
The squeeze follows a 3.6 per cent increase in the energy price cap to £1,723 on October 1.
Forecasts indicate the cap could climb a further 25 per cent in January, potentially adding £431 a year to average bills.
Nearly a quarter of bill-payers know tariffs are rising but feel powerless to stop higher charges.
More than two in five plan to wear extra layers at home rather than turn on radiators, while eight per cent expect to cut back on groceries.
Ben Gallizzi, energy expert at Uswitch.com, said: "Credit is the cushion that turns a frightening winter bill into a manageable one, but for millions, building a summer buffer for the winter simply hasn't happened.
"With the price cap forecast to jump by around a quarter in January, it is no surprise that worry is climbing.
"Nearly a quarter of people feel there is nothing they can do, but a few simple actions could make a huge difference.
"If you are deeply in arrears on your energy account and not sure what to do, speak to your supplier early and ask for an affordable repayment plan.
"Find out if you could be entitled to support.
"If you are on a standard variable tariff, it’s not too late to act.
"There are fixed deals on the market well below the predicted January cap.
"For many, it’ll also be about taking control of the energy being used.
"Smart meter households can connect to the free Uswitch app to see how much energy they use each day and where to cut back safely to manage winter bills."