Customers are less satisfied with how suppliers handle complaints, despite the number of complaints having halved since 2014.

Big six suppliers Npower and Scottish Power, alongside independent First Utility, recorded the highest proportion of “very dissatisfied” complainants.

SSE, Eon and EDF Energy performed better but failed to show any significant improvement in overall complaint handling since 2014.

The biennial survey from Ofgem compares how the largest and medium sized suppliers deal with their domestic and micro-business customers’ complaints.

However, the survey also showed that 77 per cent of domestic complainants said that it was easy to find the right contact details to make a complaint, up from 65 per cent in 2014. Following the initial contact, the experience deteriorated and 42 per cent of customers whose case had been closed by the supplier thought it remained unresolved.

Of the customers surveyed, 52 per cent were planning to switch as a result of their experience compared to 44 per cent of domestic complainants and 47 per cent among micro-businesses in 2014.

A majority of Npower and Scottish Power domestic customers who complained (71 per cent and 59 per cent respectively) said that they had or were planning to switch as a result of their experience.

The regulator has demanded that the worst performers – after Npower and Scottish Power – First Utility and Utility Warehouse, conduct and publish a thorough independent audit of their complaints handling procedures.

Ofgem’s chief executive Dermot Nolan has also written an open letter to all suppliers surveyed to demand improvement and asked them to respond publicly setting out how they have made, and intend to make, improvements.

In December last year, Ofgem took action over Npower’s complaints handling and fine the supplier £26 million as a result of its failings. In April this year Scottish Power had to pay out £18 million for similar failures.