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The Cruachan dam in Dalmally, Argyll

Scottish Power, one of the biggest energy suppliers in Britain, has been fined £18m for poor customer service.

The company, which has 3.2m customers, has agreed with Ofgem to pay the third-largest fine in the energy regulator’s history after attracting more than 1m customer complaints between June 2013 and December 2015.

The complaints, which followed the company’s move to a new IT system, related to poor handling of calls and billing problems, with 300,000 customers receiving late bills.

Dermot Nolan, Ofgem’s chief executive, said: “Scottish Power 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. This created frustration and worry for many customers, who also wasted a lot of time trying to contact the supplier by phone.

“The £18m payment sends a strong message to all energy companies about the importance of treating consumers well at all times, including while new systems are put in place.”

Scottish Power said it would pay up to £15m to vulnerable customers, including those affected by the move to the new IT system. It will pay the other £3m to charity. The company has already paid £30m to customers in compensation.

Neil Clitheroe, head of energy retail and generation, said: “In order to upgrade our old IT systems, we invested £200m on new technology to allow us to deliver smarter digital products and services to benefit our customers.

“During the complex transition between systems we encountered a range of technical issues. This led to an unacceptable increase in complaints and reduced the quality of our customer service.

“We know we need to do more and we will continue to work hard to deliver further improvements, and ensure that our customer service standards return to being amongst the best in the industry.”

Scottish Power’s problems are the latest in a string of issues facing the “big six” energy suppliers, which have been criticised by some for not reducing prices and offering poor customer service.

Richard Lloyd, executive director of consumer group Which?, said: “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.”

In December, Ofgem fined Npower, another of the big six, £26m for “failing to treat customers fairly” — the biggest fine in its history.

The Competition and Markets Authority has accused big energy suppliers of overcharging customers by £1.7bn a year between 2009 and 2013. But it has dropped its proposal to impose a price cap for customers who are on expensive default tariffs, saying it should apply only to those on prepayment meters instead.