Ofgem is to expand an existing probe into Glasgow-based energy giant ScottishPower over its complaint handling procedures.
The regulator said it had also opened new compliance cases into First Utility, Ovo Energy and Utilita over how customers grievances have been handled.
Customer satisfaction data published today by the regulator found that the greatest improvements in satisfaction were reported among those who complained to ScottishPower and nPower, which were both rated the two worst performers among the largest suppliers in 2016 - although more than half of customers said they were still dissatisfied.
The survey of more than 3,000 complainants found that just under a third - 32 per cent - were happy with how their grievances were handled, although this was up from 27 per cent since the last poll in 2016.
Dermot Nolan, chief executive of Ofgem, said: “Although the level of satisfaction about complaint handling has increased over the past two years, it is still unacceptably low. Some suppliers need to be doing considerably more to get the basics right and provide a service their customers deserve."
He added: "We will be monitoring the level of all suppliers’ customer service performance particularly closely after announcing proposals to introduce a price cap to protect those on poor value default deals from being overcharged. We are ready to - and will - act against those who fail their customers.”
Peter Earl, head of energy at Compare the Market, said: “To receive such poor service that you have to complain is bad enough, but to then receive shoddy service during the complaints process is a real kick in the teeth. The sad reality is that too many energy firms are delivering poor customer service. It is a disgrace that to receive a resolution, so many people need to go through a hellish drawn-out complaints procedure in order to get it.
"Such poor complaints handling and customer service should act as a trigger to switch supplier. It is always worth comparing providers based on their service ratings, particularly if you have had a poor experience in the past, or are currently experiencing it first-hand.”
Ofgem stressed the move to open compliance cases does not mean it has made any findings over non-compliance. It will report back with its findings in due course.
The survey was carried out in February and April, with 3,080 domestic and 703 micro-business customers who had lodged complaints with their supplier in late 2017. It covered customers of the Big Six UK suppliers, as well as the largest of the medium-sized providers.
Ofgem recently launched a crackdown on so-called rip-off energy deals, unveiling details of a cap on standard variable tariffs to cut bills for around 11 million households by about £75 on average.
It is set to come into force this winter.
A spokesman for ScottishPower said: "We are pleased the survey shows we are the most improved company for complaint handling satisfaction , but we know we still have further to go. We will continue to work with OFGEM as we roll-out new complaint handling processes and training to further speed up our progress. Meanwhile, our overall service levels continue to improve, as shown across a range of external surveys and scorecards."