Unions are braced for confirmation of huge job losses at energy giant npower.
The German-owned firm is expected to unveil plans for up to 2,500 job cuts when it publishes financial figures today.
Union officials have warned that so many job losses would deal a "devastating blow" to the company's workforce and to communities across the country.
The job cuts will represent around a fifth of npower's 11,500-strong UK workforce, and are expected to hit areas including the north of England.
Unison general secretary Dave Prentis said: "These huge job losses will come as a devastating blow to the workforce.
"Npower has been in trouble for some time thanks to poor decision-making at the very top, and workers are now paying the price. The company's failure to invest properly in new systems has left it with one of the worst customer service records in the business.
"The news suggests that npower's German owner isn't terribly committed to its UK operations. Cutting a fifth of the workforce will leave the already struggling business in an even worse state. Now months of uncertainly lie ahead for a workforce whose morale is already at rock bottom."
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.
"It also makes a mockery of George Osborne's rhetoric about a northern powerhouse if an energy company can't maintain jobs in the region."
The firm's German owners RWE revealed last month it will slump into the red after taking a 2.1 billion euro (£1.6 billion) hit on its UK and German power stations.
It said a series of writedowns on its power stations and lower energy prices would lead the group to post an annual loss of around 200 million euro (£155 million) when it reports its results. Analysts had expected a 2015 net profit of 1.2 billion euro (£0.93bn).
Npower, which has 4.9 million accounts, has major plants in the UK including Pembroke and Aberthaw in Wales and Staythorpe in Nottinghamshire.
Along with other Big Six suppliers, the firm announced a 5.2% price cut to its standard domestic gas tariff that would affect 1.2 million customers.
Oil and gas prices have tumbled over the last 18 months as global growth slows. Brent crude is some 70% lower than its peak in the summer of 2014.
The firm's power station at Didcot in Oxfordshire collapsed last month as it was being prepared for demolition, killing one worker, with three still missing.
Unite union national officer Kevin Coyne said: "It is an inexcusable way for the company to treat a loyal workforce which has worked hard to turn npower's record on customer service around in the last year.
"We will be demanding urgent answers and assurances from npower over the coming days as we seek to protect as many jobs as possible."
Shadow energy secretary Lisa Nandy said: "This is terrible news for npower workers and another blow for the energy industry after so many North Sea and solar job losses."
Npower made no comment.
Eamon O'Hearn, of the GMB union, said: "GMB is disappointed that hard-working members, many of whom have experienced significant disruption and uncertainty over the past 12 months, are set for more uncertainty over the next 12.
"GMB members, in particular those in customer-facing roles, have been instrumental in helping the company to turn around the retail business and we deserve to know the full picture as soon as possible.
"RWE's German workforce would not be treated in this manner, and it is fuelling concerns that this announcement was leaked to satisfy City traders without any thought for the impact on the morale of dedicated members.
"GMB hopes that is not the case and believe the company now has an uphill battle to ensure that this perception isn't a reality.
"GMB stands ready to work with the company to continue to provide professional jobs for thousands of members across the UK."