Britain's Big Six energy utilities are driving nearly half of their customers to seek alternative suppliers because of a poorer handling of complaints, energy regulator Ofgem said.
The findings of an Ofgem survey further dent the image of the supply industry after a two-year investigation found that providers have overcharged customers because of uncompetitive standard energy tariffs.
While complaint numbers have halved since 2014, those who do complain are significantly less satisfied with the response, citing agents breaking follow-up appointments and utilities' failure to resolve issues, among other issues, Ofgem said.
On Thursday supplier E.ON paid 3.1 million pounds to customers and energy charities after its agents missed appointments.
The worst performers in Ofgem's survey were npower, Scottish Power and smaller peer First Utility due to the highest proportion of "very dissatisfied" claims, it said.
Particularly poor complaints handling by npower and Scottish Power led to 71 percent and 59 percent of complainants, respectively, saying they had or were planning to switch as a result of their experiences.
Nearly two-thirds of medium-sized suppliers' complainants said the same.
Even suppliers that fared best - SSE, E.ON and EDF - failed to show significant improvement in complaints handling over the past two years, Ofgem said.
"More customers are no longer prepared to put up with poor customer service and 52 percent of all complainants surveyed had or were planning to switch as a result of their experience," according to the watchdog's survey of residential and micro-business bill payers.
Only a third of those surveyed were given a named contact at their supplier, Ofgem said, while forty-two percent of complainants whose cases had been closed by the supplier thought it remained unresolved.
(Reporting by Oleg Vukmanovic in Milan, editing by William Hardy)