By Jonathan Davies
Scottish Power will be banned from adding new customers unless it improves its customer service within three months.
The energy regulator Ofgem said the energy supplier must improve the time it takes to answer customers calls, reduce its backlog of bills and act on ombudsman rulings quicker.
If Scottish Power does not meet all of the targets, Ofgem will ban "proactive sales activities".
"Scottish Power's customers are experiencing unacceptably long call waiting times and a drastic drop in overall customer satisfaction," said Sarah Harrison, of Ofgem.
"In a properly functioning market we would expect companies to compete keenly on service. The need for our intervention here is yet more evidence that the energy market is not working for consumers."
Neil Clitheroe, chief executive of Scottish Power retail and generation, said: "We unreservedly apologise for these problems. It is simply not the high service standard long associated with ScottishPower, nor the customer service standards industry regulator expects.
"We are determined to put it right. We are working hard to make it easier for our customers to contact us and to answer every call, email and complaint as soon as we can."
Scottish Power said it will hire an extra 250 staff, on top of the 450 it already hired earlier this year, in order to cope with the demands.
It will update Ofgem on its progress on a monthly basis.
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