Energy giant Npower is set to cut around 2,500 jobs in the UK - a fifth of its workforce in the country.
With an official announcement expected to be made to staff this week, Npower, which is owned by German energy firm RWE, reported a loss of £48 million for the first nine months of the financial year. And it is expected to report a loss for the full year, when figures are announced on Tuesday.
2015 saw it lose around 200,000 customers in the UK and it is understood to receive more complaints than any other 'big six' energy supplier. In December, Npower agreed to pay a £26m settlement - the largest ever ordered by the UK energy regulator - over inaccurate bills and failure to deal with complaints properly.
It is understood that the majority of the job cuts will come in its sales and marketing departments.
Eamon O'Hearn of the GMB union said: "Many hard-working staff have already been outsourced and so any further job losses for directly and indirectly employed staff would be another kick in the teeth for communities."
Last month, Npower joined the rest of the 'big six' in reducing gas prices. Npower customers will see their bill fall 5.2%, or £32 a year for the average household.