Npower lost almost 355,000 customer accounts in 2015, the firm has revealed, as it announced it is axing 2,400 jobs across the country.
Customer gas and electricity accounts dropped by around 7% to 4.77 million by the end of 2015 because of "over-complicated processes" and poor customer service.
Problems with its IT billing system saw npower issue more than 500,000 late bills between September 2013 and December 2014.
This, along with poor complaints handling landed the provider - owned by German energy group RWE - with a £26 million fine from regulator Ofgem.
Npower power said it is beginning to see improvements in its customer service, with complaints per 100,000 customers more than halving last year and coming in below the industry average in the fourth quarter.
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