ScottishPower is to pay an £18m fine for failing its customers by botching an IT system upgrade.
The fine is the third-biggest ever imposed by watchdog Ofgem, which said the company, one of the UK's "big six" energy providers, had not adequately handled customers' calls, complaint resolution or billing.
However, the regulator praised ScottishPower for cooperating with its 18-month-long investigation and for improving its service in the meantime.
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A new IT system led to more than 300,000 people receiving late bills and having to call the firm, often unsuccessfully, to resolve problems.
"ScottishPower let its customers down during the implementation of a new IT system. When things went wrong, it didn't act quickly enough to fix them," said Ofgem chief executive Dermot Nolan.
"This created frustration and worry for many customers, who also wasted a lot of time trying to contact the supplier by phone."
Nolan added that the fine, which will be paid to vulnerable customers and charities, sends a "strong message to all energy companies about the importance of treating consumers well at all times". In December, the regulator ordered Npower to pay £26m after it sent inaccurate bills and mishandled complaints.
ScottishPower's Neil Clitheroe apologised "unreservedly" to all customers affected by the problems and admitted there had been "an unacceptable increase in complaints and reduced … quality of customer service".
He added that no customer would be left out of pocket by the errors and said the company was still in the process of paying compensation, the BBC reported.
