Emergency phone calls may not be possible in parts of France if there are prolonged power cuts this winter, the CEO of telecoms operator Orange said on Wednesday, as the country braces for energy rationing amid the war in Ukraine.

The stark warning from Christel Heydemann comes in contrast with the French government's reassuring tone on the matter and highlights the challenges the telecoms industry faces in keeping mobile services running in the event of prolonged power cuts.

"If, in a geographical area, (mobile network services) are turned off for two hours, there will be no access to the emergency number services for a while," Heydemann told French lawmakers at a hearing held in the Senate.

"It is illusory to imagine that we'll be able to maintain service to all French people in the event of power cuts," she said. (Reporting by Mathieu Rosemain; editing by Josephine Mason)