Energy giant Npower has announced plans to restructure its UK business, which union sources fear will lead to the loss of up to 4,500 jobs and the closure of a number of call centres.
The news was described as a “body blow” by one union official, especially coming just weeks before Christmas.
Workers will be given details at briefings later on Friday.
Johannes Teyssen, chief executive of Npower’s owners E.ON said: “The UK market is currently particularly challenging. We’ve emphasised repeatedly that we’ll take all necessary action to return our business there to consistent profitability.
“For this purpose, we’ve put together proposals and already begun discussing them with British unions.”
Chris Jukes, of the GMB union, said any job losses would be a blow to workers, particularly this close to Christmas.
E.ON said it was stepping up its "ambitious cost-cutting efforts" without losing sight of its customers, adding: "This is based on leaner, increasingly digital processes that also improve the customer experience."
An E.ON spokesman said: "It is only right and fair that colleagues are informed first of any possible changes and we will be making no further comment at this time."
Unison general secretary Dave Prentis said: "This is a cruel blow for Npower employees. They've been worried about their jobs for months. Now their worst fears have been realised, less than a month before Christmas.
"The UK energy market is in real danger of collapse. If nothing is done, there could soon be other casualties.
"Npower's demise means there's no time to waste. It makes the powerful case for bringing the retail arms of the Big Six energy firms into public ownership.
"This would preserve jobs, ensure customers get a better deal and allow the UK to meet its carbon neutral targets."