Energy supplier Scottish Power will be banned from signing up new customers unless it resolves serious customer service issues, regulator Ofgem has announced.
The company - one of the UK's Big Six energy firms - has agreed to try to meet three targets set by Ofgem to drastically improve its customer service within three months or face being prevented from undertaking sales activities.
Scottish Power must "significantly" reduce the time in which it answers customers' phone calls and take action to reduce the number of late bills for customers by more than half from 75,000 today to 30,000 by the end of December.
It must also completely remove the backlog of complaints that have been referred to the by the Energy Ombudsman.
The firm will be required to publish weekly progress reports detailing call waiting times from the end of November, while reports on late bills and ombudsman complaints are to be published on its website.
If any targets are missed, Scottish Power's sales activities must be suspended immediately.
The news comes as price comparison site uSwitch found that Scottish Power is the worst of the Big Six when it came to overcharging, with one in six of its customers claiming they were sent an incorrect bill in the past 12 months.
Sarah Harrison, senior partner in charge of enforcement at Ofgem, said: "ScottishPower's customers are experiencing unacceptably long call waiting times and a drastic drop in overall customer satisfaction. Ofgem's action today will drive immediate improvements for ScottishPower customers. 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. This further justifies our decision to refer the market to the Competition and Markets Authority."
She added: "The investigation we launched today is the third time we have used our new standards of conduct to make suppliers treat consumers fairly. This is a clear signal to suppliers to up their game and give consumers the service they are entitled to."
In a letter to Ofgem, Scottish Power chief executive, Neil Clitheroe, said: "I would like to apologise unreservedly to any customer who has not received the level of service that they deserve from Scottish Power in recent months and provide my personal assurance that we will do what we can to correct every problem, pay appropriate compensation and ensure that no customer is disadvantaged."
Ofgem made similar demands of Npower in the summer, also over serious billing issues.