npower's chief executive has apologised "unreservedly" after billing errors affected thousands of customers.
Paul Massara wrote in a letter to npower's 3.4 million customers: "We've let many of you down recently in the overall levels of customer service we've been providing. We apologise unreservedly."
He said the billing problems arose after customer details were transferred on to a new computer system. Around 700,000 customers are thought to have been affected by the problems, though it is believed the vast majority are not financially worse off.
Mr Massara said the issue was being dealt with as "our top priority", and hundreds of staff were working to address it.
Many customers affected had already been contacted individually to address specific problems and others would be, he added.
More top news
Bank Holiday bursts of rain for some
President Trump has called the North Korean leader a "smart cookie", for managing to assume power after taking office at such a young age.
US dance duo The Chainsmokers surprised hundreds of students by gate-crashing their prom in the US.