Energy market regulator Ofgem said it has opened compliance cases into First Utility, Ovo Energy and Utilita and is expanding a recent case on Iberdrola ScottishPower.

The regulator also said it was requiring all other domestic suppliers surveyed - Centrica British Gas, Innogy SE Npower, Utility Warehouse, SSE, EDF's EDF Energy, E.ON and Co-operative Energy - to provide improvement plans on how they will deal with complaints and provide appropriate updates.

The survey of around 3,100 complainants - carried out every two years - found that satisfaction has improved since the last survey in 2016, with a third (32 percent) of domestic customers satisfied with how their complaint was dealt with, an increase of five percentage points from 2016, Ofgem said.

However, it also found that the proportion of customers who are dissatisfied (57 percent) remains higher than those satisfied with how their complaint had been dealt with.

The main reasons for dissatisfaction were the length of time taken to resolve an issue, not being kept up to date with the progress of the complaint and suppliers not providing complainants with a clear idea of how long a resolution would take.

"Although the level of satisfaction about complaint handling has increased over the past two years, it is still unacceptably low," Dermot Nolan, chief executive of Ofgem, said in a statement.

"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," he added.

(Reporting by Nina Chestney and Arathy S Nair in Bengaluru; Editing by Toby Chopra)