ScottishPower has been fined £18 million by regulator Ofgem for customer service failings.
The energy firm "failed to treat its customers fairly" when handling calls, dealing with complaints and billing, the watchdog said.
Dermot Nolan, Ofgem 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."
ScottishPower was bl