Ofgem has called on energy suppliers to "act quickly" to resolve the overcharging of customers on pre-pay gas meters and warned it had not ruled out taking action.
A spokeswoman said: "We are determined that no consumer should lose out because some energy suppliers have overcharged some consumers on pre-payment meters.
"By the end of this week we expect industry to come forward with a swift timetable for resolving all traceable cases of over-charging customers, repay consumers within a clearly-agreed and publicly-announced timetable, reconfigure meters as quickly as possible and protect customers from over-payment in the meantime.
"Suppliers must act quickly to resolve the problem in a way that treats customers fairly. We have not ruled out enforcement action."
Many customers with faulty pre-payment gas meters have been overcharged for years, ITV News understands.
ITV News' Consumer Editor Chris Choi said some of the meters which have been found to be faulty had been in homes since 2007:
1.5 million gas meters are faulty and many have been overcharging people for years energy industry confirms. Those with pre-pay meters hit.
"Another energy scandal" say consumer groups. On average customers have been overcharged up to £7 a year - not a fortun but the principle.
Up to 1.5m pre-payment gas meters have been found to have been overcharging customers due to a fault.Read the full story ›
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.
Power generated from fossil fuels should be phased out completely by 2100 a new report has warned.
The United Nations climate body published the final report of its latest assessment on the science of climate change, drawing together three studies published in the past year.
In the majority of low?concentration stabilization scenarios, the share of low?carbon electricity supply (comprising renewable energy (RE), nuclear and Carbon Capture Schemes (CCS), including BECCS) increases from the current share of approximately 30% to more than 80% by 2050, and fossil fuel power generation without CCS is phased out almost entirely by 2100.
The report comes as efforts build towards securing a new global treaty on climate change, which it is hoped can be agreed in Paris at the end of next year, and it said that international co-operation was "critical" for effective efforts to tackle the problem.
One of the Big Six energy companies under fire for poor customer satisfaction has defended its actions and pointed to the installation of a new IT system for its poor ratings.
Scottish Power, which saw its customer satisfaction rate drop by more than half, said:
The installation of a new £200 million IT system throughout 2014 has led to this sudden drop in our customer satisfaction scores.
All customers have now been migrated on to the new system, which has resulted in a very busy period as accounts have been brought up to date.
Although the transition has been challenging, we have recruited more than 250 additional customer service staff to answer calls and resolve any customer complaints.
We are confident that customers will now start to see real long-term service improvements coming through. We now have the longest call centre opening hours in the industry and have also been expanding our online services.
Energy companies have been warned of the "unacceptable" customer satisfaction levels and told to "act quickly" in a stern letter from Ofgem.
The energy regulator wrote to all companies after a damning report into consumer satisfaction levels found:
- Satisfaction with Npower and Scottish Power had fallen particularly markedly, from 36% to 21% and from 44% to 20% respectively.
- Only SSE managing to maintain levels of satisfaction found two years ago.
- In today's letter, Ofgem warned companies to improve the speed of resolving complaints, communicate better with customers during the process and be more proactive in finding a solution.
Ofgem has written to the Big Six energy companies to demand they improve their poor handling of customer complaints, it has emerged.
The energy watchdog also sent letters to smaller and independent companies, and voiced concerns about the "industry-wide failure" to work with fed-up customers.
In his letter Ofgem chief executive, Dermot Nolan, told companies to improve the speed of resolving complaints, communicate better with customers during the process and be more proactive in finding a solution.
It follows research by Ofgem that found more than half of those who had complained - 57% of domestic customers and 52% of small businesses - were not satisfied with how their supplier handled the problem.
And in what appeared to be a total breakdown of communication - in almost half of cases where the supplier considered the case resolved, the customer did not.
An expert in complaining to companies has encouraged disgruntled customers to write a letter instead of calling customer service if they want results.
Jasper Griegson told Good Morning Britain: "You have got a paper trail, you're not wasting your life and best of all, you're not listening to Vivaldi."