ENERGY giant ScottishPower has been ordered by regulator Ofgem to improve the way it deals with customers or face a sales ban.

It found the firm was making customers wait too long on the telephone, forcing a quarter of calls to be abandoned, while many bills were being sent late.

Ofgem warned the Spanish-owned company that failure to meet new targets would result in it having to suspend sales.

The firm, one of the Big Six suppliers, has been told to improve average call waiting times to two minutes, by the end of January, without any worsening in the level of calls abandoned by customers.

Scottish Power has also been ordered to reduce the number of overdue bills from of 75,000 to 30,000 by the end of December.

Ofgem also said a backlog built up by the company on dealing with decisions by the ombudsman must be wiped out by the end of this month.

Sarah Harrison, senior partner in charge of enforcement at Ofgem, said: "Scottish Power's customers are experiencing unacceptably long call waiting times and a drastic drop in overall customer satisfaction

"Ofgem's action will drive immediate improvements for Scottish Power customers."

She said the need for intervention was further evidence the energy market was not working for consumers and backed up its decision to refer the sector for a full-scale probe by the Competition and Markets Authority.

"The investigation we launched 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 apologised "unreservedly" to any customer who had received sub-standard service in recent months.

He said: "We will do what we can to correct every problem, pay appropriate compensation and ensure that no customer is disadvantaged."

Scottish Power will each month have to publish its progress towards the new commitments and any missed ta rgets will mean proactive sales activities have to be instantly suspended, Ofgem said.

Shadow energy secretary Caroline Flint said: "This is yet another example of the big energy companies failing their customers.

"Under David Cameron people are paying more than ever for their gas and electricity, but all too often the customer service they receive is not good enough."