Energy giant SSE revealed it lost another 50,000 gas and electricity accounts as small suppliers continue to put their Big Six rivals under pressure.
The group said UK and Ireland electricity and gas customer accounts fell to 8.16 million in the three months to the end of June, down from 8.21 million at the end of March.
It comes after rival ScottishPower revealed on Wednesday it lost around 40,000 customers year-on-year in the first six months of 2016, falling to 5.4 million gas and electricity accounts.
Households are increasingly turning to smaller suppliers amid a backlash against the major players, with consumers complaining the sector is slow to pass on wholesale energy cuts and offers poor service.
The Competition and Markets Authority has found that households stuck on more expensive “default” standard variable tariffs are overpaying to the tune of £1.4 billion a year.
British Gas lost 224,000 residential customers in the first three months of the year after mounting competition from smaller suppliers hit the group hard.
The UK has a total of just under 40 energy suppliers as new rivals have entered the market in recent years.
But despite the customer losses, SSE insisted it had made a “solid start” to its new financial year.
It also confirmed plans to sell up to a third of its 50% stake in regional gas distribution business SGN, with a sales process now under way.
Energy analysts at Jefferies said SSE remained under pressure, despite the rate of customer losses in its first quarter easing back from a 4% drop in its previous financial year.
“With competition in the retail market remaining high, the outlook for SSE’s retail business remains challenging,” they said.
An SSE spokesman said: “It is an intensely competitive market with more than 30 suppliers competing for customers.
“It’s great for customers as they have more choice than ever before. We are working hard to attract and retain customers.”