Centrica, which owns British Gas, is planning to slash 700 management and back office jobs as it faces "growing challenges".
The firm said staff had been told of the news, part of a previously-announced plan to cut the workforce by 4,000.
"This difficult decision was made because we need to respond to the growing challenges we face," a spokesman said.
"The energy market is going through continued rapid change, competition is fierce, our energy customers are leaving us and we're operating under a price cap."
But unions described the job losses as a "terrible blow" for the workforce.