Npower has written to customers apologising for recent billing problems and promised that customers will not lose out financially as a direct result.
Last month, the energy giant topped a list of complaints compiled by watchdog Consumer Futures, which branded the company's performance as "unacceptable".
Recent problems encountered by customers include billing errors, such as direct debits not being set up properly and difficulties in accounts being created.
Long waiting times when trying to phone the company added to customer frustration.
In its letter, the company admitted it had let many of its customers down in recent months, and apologised "unreservedly". It blamed the problems on the transfer of customer accounts to a new computer system.
Paul Massara, npower's chief executive, said: "In recent months, we've let many of our domestic customers down and I wanted to write to each of them personally to say sorry.
"Although we've made good progress fixing many of the underlying issues, we still have a long way to go, and our customers deserve the best possible service."
As a gesture of goodwill, the energy giant also said it will pay £1 million to vulnerable customers via the Health Through Warmth fund, which helps homeowners with cold-related illnesses who need heating repairs and installations but cannot afford them.
Adding salt to customers' wounds, they have seen the biggest price hike from any of the Big Six energy providers this winter, with bills rising by an average of 10.4% - adding £141 to the average bill.