Npower is to cut 2,400 jobs after reporting losses of more than £100 million.
The energy giant said it will make "extensive cost savings" to help turn around the "significant" losses made in 2015 and return the company to profitability.
The German-owned company said its 11,500 employees will be reduced by 2,400, through a mix of those who work directly and indirectly for npower.
A two-year recovery plan was announced to deliver a "robust business" built on lower costs, simplicity, high-quality customer service, and ready for the challenges of the future.
A statement said: "Npower takes its responsibilities to its employees seriously and will consult fully with affected employees and with unions over its proposals for the future of the business."