Energy supplier E.On has agreed to pay more than £650,000 to its customers after it left more than a million households out of pocket when it charged them early before Christmas Day.
The mistake led to 1.6m customers, who expected to be charged at the beginning of January, having their energy bills charged on Christmas Eve.
It meant that customer may have been forced to unexpectedly go into their bank overdrafts over the Christmas period.
The energy company noticed the issue on December 23 and told industry regulator Ofgem. E.On then refunded most customers on December 29 and December 30.
However on New Year's Eve, E.On discovered another 10,060 customers had also been affected.
Ofgem said that discovering the extra customers had been impacted so late suggested the company did not act properly to put things right.
Anna Rossington, Ofgem's director of retail, said: “Ofgem expects suppliers to adhere to the terms of contracts they have with customers, in particular the agreed direct debit payment dates.
“This failure is a reminder to suppliers that, when making changes to their systems, they need to undertake appropriate checks to avoid any unintended consequences for customers.
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“Ofgem is always prepared to work with suppliers who have failed to comply with their obligations but who have self-reported and are determined to put things right, as E.On has done.”
The problem occurred when E.On tried to change its "friendly credit hours". These hours are a system that has been designed to make sure that households with pre-payment meters are not left without energy when shops are closed during the holidays.
E.On estimated that the error could have cost customers £427,312. It has agreed to pay this sum plus an extra £200,000 into Ofgem's voluntary redress fund, which was set up to help vulnerable customers in the 海角视频.
E.On has also made £55,039 in redress and goodwill payments to customers who faced bank charges or out-of-pocket expenses because of the problem.
E.On chief executive Michael Lewis said: “This error should not have happened and it was unfortunate that it was so close to Christmas.
"We apologised to those affected at the time and I apologise to them again now. As soon as we noticed the issue, we took immediate steps to put things right for our customers.”