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.
Ofgem 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.
Problems with billing systems can "turn households budgets upside down" and energy companies owe it to their customers to make any changes as smoothly as possible.
Citizens Advice chief executive Gillian Guy said:
The number of complaints about npower and Scottish Power doubled in the first quarter of 2014, according to Citizens Advice.
Citizens Advice and Citizens Advice Scotland point to problems with new billing systems as to why there was a sharp rise in complaints.
Complaints about npower rose from 306.8 for every 100,000 customers during the closing months of 2013, to 592.4 between January and March - or one complaint for every 168.8 customers.
Scottish Power complaints increased from 100.5 per 100,000 customers to 197.7 in the first quarter.
All suppliers saw an increase in complaints, although the rise was only slight for some, it added.
ScottishPower said it will "try and avoid further price rises in 2014" but stressed this will "depend on whether there are increases in wholesale energy prices or other costs outside our control".
The energy firm said the taxpayer funded £12 rebate will be applied to all households "in line with the Government timetable" of two years.
Retail chief executive Neil Clitheroe said, “We welcome all of these changes proposed by Government, especially those to the ECO programme, as they represent a strong commitment to have a more rational debate on energy policy in the UK".
Following the Government's confirmed shake-up of green levies, ScottishPower is set to make an announcement:
Keith Anderson, the Chief Corporate Officer for Scottish Power, told ITV News that the targets set in today's Energy Bill for a 40% electricity reduction by 2020 were "achievable".
"What's been announced today really sets the framework to allow the industry to come forward with the investment the country needs to deliver a low-carbon future for the UK.
"Those targets are very ambitious but certainly achievable."