German energy giant EON on Friday said it planned to restructure its UK division Npower, sparking concern among British unions that up to 4,500 jobs could be cut.
"The UK market is currently particularly challenging," EON chief executive Johannes Teyssen said in a statement.
"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," Teyssen added.
EON said its planned restructuring over two years would cost the company 500 million pounds (USD 645 million, 586 million euros).
Unions said call-centre jobs were particularly at risk.
Dave Prentis, head of British union Unison, described the announcement as "cruel", adding that as many as 4,500 jobs could be axed.
"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," he added in a statement.