ScottishPower, npower and First Utility are the worst firms to handle customer complaints.

That’s according to Ofgem which surveyed more than 3,000 Big Six and Utility Warehouse, First Utility and Ovo Energy customers as well as more than 460 micro-businesses that complained at the end of last year.

A majority of npower and ScottishPower’s domestic customers who complained said they had or were planning to switch as a result of their experience as did 63% of medium sized suppliers’ complainants, it added.

However, the regulator stated that despite the total number of complaints having halved since 2014, customers are less satisfied with how their firm handles it compared to two years ago.

The survey shows suppliers that fared best in 2014– SSE, E.ON and EDF Energy – failed to show any significant improvement in overall complaint handling in the last two years.

A total of 42% of complainants whose case had been closed by the supplier thought their problems remained unresolved and 52% of all of them had or were planning to switch as a result of their experience.

Dermot Nolan, Chief Executive of Ofgem said: “Improving communication at the outset and throughout the complaint journey is necessary to prevent consumers feeling they need to manage the process themselves and to improve satisfaction. We would like you to publically respond to this letter setting out how you have and intend to improve the complaints handling experience you give your customers and specifically how you will ensure your communication with complainants improves.”

Commenting on Ofgem’s report, Lawrence Slade, Chief Executive at Energy UK said: “Energy suppliers are focused on improving service to their customers. Halving the numbers of complaints made in two years is strong evidence of that.

“However, it is also clear that the industry needs to keep on improving the service it provides to customers so we not only continue to reduce the number of complaints but we raise the levels of satisfaction as well.”