Energy customers with estimated bills could be racking up debts of hundreds of pounds without realising it, according to Citizens Advice.
The charity says thousands of people have got in touch over their estimated gas and electric bills.
In the worst cases people have found they owed over £1,000 when they thought their direct debits were covering their energy costs.
It blamed the problem on spiralling tariffs and billing issues.
Actual usage
Tony Herbert, a senior policy officer with Citizens Advice, said with energy costs forecast to rise further this winter, the problem is likely to get worse.
"Families can rack up huge fuel bills without even realising it because the amount they're paying is actually based on estimated readings."
"We're a family we have bills to pay so every penny counts at the moment"
"They might have a direct debit set up; they might think they're paying their bills," he said.
"But because they're paying on estimated readings it may not reflect how much they're actually using."
The BBC learned last month that household energy bills could increase by as much as 40% this winter as oil and wholesale gas prices hit record highs.
The increases could mean households paying £400 more a year on average for their gas and electricity, senior industry sources said.
'Big jump'
Richard Mann is one gas customer who has found out how inaccurate estimated bills can be.
For the last two years, all his gas bills have been estimated.
His gas supplier originally set his direct debit at £15 a month, but then reduced it when his account was in credit.
However, last month he received a bill saying he owed £238 and that his direct debit would have to rise ten-fold to pay it.
"It's a big jump and we have to find that money from somewhere," he said.
"We're a family, we have bills to pay so every penny counts at the moment."
Learning experience
Hundreds of consumers have written to the BBC News website with their stories.
Christine Shelbourne from Chipstead in Surrey said she had become accustomed to paying her bills by direct debit and the energy company coming to read the meter once a year, resulting in what she called an annual "reconciliation bill".
In December, she received a bill for £1,500 - the energy company said the readings from the previous four years had not been taken into account.
"I managed to negotiate a 35% reduction in the payment and now read my meter and send the details off via the internet each quarter", she said.
"It is important that we actively start managing our energy"
Tony Ayre from Minehead, Somerset, knew something was wrong when he received a £40 electricity bill for the entire winter.
He provided a meter reading, which showed he had used 10 times the estimated amount and his direct debit rose from £15 a month to £60 a month.
"Now, I give meter readings every couple of weeks online to ensure it doesn't happen again," he said.
Consumer watchdog Energywatch says a third of all bills are estimated and therefore could be wrong.
"They're not based on real meter readings, they're not based on real consumption data," said Energywatch's Adam Scorer.
"You can just imagine the problems that will come out from the fact that a third of bills in Britain are by and large wrong."
Billing complaints
However, the Energy Retail Association, which represents the gas and electricity companies, said that the majority of the 200 million bills the industry sends out each year are correct.
"It is hardly surprising that the majority of complaints or enquiries that anyone gets are going to be about billing, because that is the way that most energy companies will actually touch their customers," said Duncan Sedgwick, the association's chief executive.
Energy expert Ann Robinson, from the internet price comparison site uSwitch.com, urged people to stop being "complacent" and take responsibility for reading their own meters.
"It really is important that we actively start managing our energy," she told the BBC.
She said consumers often do not realise that "if the companies read the meters more, it has a cost and energy bills will go up".
"If people have problems reading the meters, say a disabled person, then the companies will consider moving the meter or will come more frequently to read the meter," she added.
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