Energy giant SSE lost 430,000 customers last year as competition intensified and it prepares to merge its retail business with npower.
It is the latest among the Big Six energy companies to report customer defections as energy prices rise and the government introduces new caps on bills.
British Gas lost 110,000 customers in the first four months of this year and npower saw an exodus of 120,000 customers in the first quarter of 2018.
SSE said its domestic accounts fell from 7.23 million to 6.8 million.
The company made the announcement as it reported a 6% fall in adjusted pre-tax profit of £1.45bn in the year to March 31. Unadjusted, profit slumped almost 39% to £1.08bn.
Revenue rose 8% to £31.23bn.
Despite the defections, the so-called “Beast from the East” cold snap helped it to maintain a flat operating profit of £260.4m at its household supply business.
“While electricity tariffs increased to recognise rising non-energy costs, overall profits were also impacted by customer account losses and the introduction of price caps for certain customer groups, offset by ongoing efficiency savings,” the company said in a statement.
“The business also benefited, in the last quarter of the financial year, from higher customer energy consumption due to unseasonably cold weather.”
SSE took a charge of £213.3m for its plans to merge its retail business with npower.
The combined business, which will be 65.6% owned by SSE shareholders and 34.4% by npower’s owner Innogy, will be listed on the London Stock Exchange.
The deal has yet to clear regulatory scrutiny.