Npower is slashing 2,500 jobs, more than a fifth of its UK workforce. Owned by the German energy giant RWE it employs 11,500 people in the UK and is one of the country's big six gas and electricity suppliers.
The company posted a loss of £48m for the first nine months of last year.
It has also lost about 200,000 UK customers and received the most complaints of the six biggest energy suppliers in 2015.
In December it was ordered to pay a settlement of £26m - the biggest payment ever demanded by UK energy regulators - for sending out inaccurate bills and failing to deal with complaints correctly.
One worker from the sales team said he was not surprised to hear that job losses are likely to be announced this week. He claimed that he had been told to look for other jobs since December, when the £26m settlement was ordered.