Energy firm Powergen has admitted that some its customers have received "frankly appalling" service from the company's call centres.
A huge bill for one Powergen customer
The admission came after complaints from hundreds of Powergen customers in Eastern England about their bills.
Powergen said a few mistakes had slipped through the net and admitted that some of its customer service staff had not been properly trained.
More than 500 people have told the BBC that they have been charged too much.
The energy firm has now started ringing customers whose problems had been passed on to it by the BBC, to see if it could resolve the complaints and offer compensation "on a case by case basis, where it is appropriate".
One such customer is Dave Robertson, from Cromer in Norfolk, who received an offer of £50 from Powergen this afternoon.
Back in June he had faced an erroneous bill for £630.26, along with a threat that bailiffs would be sent in if he did not pay.
Two days ago Powergen wrote admitting he was in fact £50 in credit.
Problems with excessive and inaccurate bills first came to the attention of the BBC News programme Look East last Friday.
Since then the programme has been inundated with messages from viewers describing their surprise and shock at receiving unexpectedly large electricity and gas bills.
One customer, Hilary Bayfield, from Happisburgh on the Norfolk coast, received a bill for more than £59,000.
Speaking to BBC Radio Five Live she said she was unhappy with the response when she complained about the bill.
"When they do say they will call back," she said.
"It would be nice if somebody actually, physically, did call us back.
"An apology would be very nice from the point of view of the upset and hassle this bill originally caused."
Most of the complaints have been about much smaller bills - typically ranging from £1,000 to £3,000.
Powergen responded by apologising for any bills with mistakes.
It explained that any errors may have been caused by meter readings that were wrong, or by its computer billing system not being able to cope with readings taken from new meters displaying 6-digit rather than 5-digit numbers.
Powergen also pointed out that the number of complaints was very small in comparison to the 30 million bills it sends out to its customers each year, and added that 99.9% were right.
But its customer service director, Paul Elliot, told Radio 5 Live that something had clearly gone wrong.
"We do have procedures in place that should check anomalies like this, bills that are extraordinarily large for residential customers. In this case it simply slipped through the net."
And he admitted: "Somebody didn't do their job properly and a bill went out that should have been corrected."
Consumer watchdog Energywatch said it had been receiving about twice as many complaints about Powergen than the industry average.
Energywatch believes that the firm's problems stem from the purchase of a rival energy supplier, TXU, in December 2002.
This led to details of the newly acquired customers being transferred to another computer database.
"We got lots of complaints around July which weren't being cleared quickly enough," said Robert Hammond of Energywatch.
"We think the backlog of excessively big bills being thrown up got so bad they sent them out anyway, and waited for customers to complain."
As well as complaining about huge bills, many of those who have contacted the BBC have commented on the poor response of staff at the company's call centres.
These are in Leicester, Nottingham and Rayleigh in Essex.
Many said they had failed to receive any sort of helpful response.
About this, Powergen's Mr Elliot was scathing.
"The standard of customer service that some of these customers were getting is frankly appalling, it is just not good enough," he said.
"I think the training we put in place for some of our people wasn't effective, and we haven't encouraged the right standards of customers service."