ScottishPower has become the latest utilities provider to be hit with a huge fine over poor customer service.
The company said this morning it had reached a deal with Ofgem, the utilities watchdog, to pay £18m - £15m of which will go to vulnerable customers, and £3m of which will go to charity.
In a statement this morning, the company said it had been related to the implementation of a £200m IT system.
Neil Clitheroe, ScottishPower's chief executive, explained: "During the complex transition between systems we encountered a range of technical issues. This lead to an unacceptable increase in complaints and reduced the quality of our customer service."
“I gave a guarantee that no customer would be left out of pocket by these issues and we continue to compensate customers who have been affected," he added.
"Additional customer service advisors were added and we extended our opening hours to the longest in the industry in order to improve our customer service levels."
But Juliet Davenport, chief executive of independent rival Good Energy, criticised the "Big Six" energy providers.
“[They] have been taking advantage of loyal customers for far too long. There are better deals and higher levels of quality service to be found elsewhere.
Meanwhile, Which? executive director Richard Lloyd said the company had "failed its customers on the very basics".
“It’s no surprise they have received this fine from the regulator. Customers were poorly treated for years and, while this penalty is welcome, you have to question whether it will act as a strong enough deterrent to others.
“With poor customer service too often the norm, it’s high time the large energy firms put the basics right for their customers by sorting out shoddy service and complaints handling.”