Big Six energy supplier Scottish Power was today 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 for too long on the telephone, forcing a quarter of calls to be abandoned, while too many bills were also being sent out late.
Ofgem warned the Spanish-owned firm that failure to meet new targets would result in it having to suspend sales activities. It has separately launched an investigation into the gas and electricity supplier's treatment of customers.
The firm 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 the current level 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 today will drive immediate improvements for Scottish Power customers."
She said the need for intervention was further evidence that 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 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 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 targets will mean proactive sales activities have to be instantly suspended, Ofgem said.
The regulator took action earlier this year against rival npower, ordering the supplier to put right billing and complaints issues or face a telesales ban. In September, it announced npower had met targets.
It came as figures from the energy ombudsman service showed that it received 5,011 complaints in October, the second highest figure on record.
The total was a 9% increase on September and nearly three times as many as the 1,850 complaints in October last year.
The vast majority of complaints related to billing problems, such as disputed charges, inaccurate invoices and the failure in some cases to send out any bill at all.
Energy UK figures this week showed customers switching away from the Big Six - British Gas, SSE, Scottish Power, npower, E.ON and EDF - towards smaller suppliers.
Chief ombudsman Lewis Shand Smith said: "While October's figures have shown that consumers are ever more willing to escalate complaints when they feel like they have been wronged, it's particularly encouraging to see more consumers willing to vote with their feet by going out and finding the best deal for them."
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."