Workers at French nuclear power stations are due to down tools on Thursday amid growing industrial action over controversial labour reforms.
The CGT union said staff at 16 of France’s 19 nuclear plants had voted for a one-day strike.
The government said on Wednesday it was dipping into strategic oil reserves as strikers blockaded refineries.
Unions want the government to reverse controversial labour reforms forced through parliament earlier this month.
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France’s state-run power company, Electricite de France, declined to comment on how Thursday’s one-day strike at nuclear plants would affect supply.
Nuclear power provides about 75% of the country’s electricity.
Strikes and blockades are already disrupting six of France’s eight oil refineries.
Clashes broke out at one refinery on Tuesday when police broke up a blockade at Fos-sur-Mer in Marseille.
Workers at a large oil terminal in the port of Le Havre were due to go on strike on Thursday to block imports.
Transport Minister Alain Vidalies said 40% of petrol stations around Paris were struggling to get fuel.
Motorists have been panic buying to avoid shortages.
President Francois Hollande told ministers on Wednesday that “everything will be done to ensure the French people and the economy is supplied”.
Analysts say France has nearly four months of fuel reserves.
Industrial action also spread to France’s railways on Wednesday, with a strike by train drivers cutting some high-speed TGV services as well as regional and commuter trains. More transport disruption was expected on Thursday.
The CGT has also called for protest rallies in cities across France.
The BBC’s Lucy Williamson in Paris says the escalating action is raising concerns for the Euro 2016 football championships due to begin in France in just over two weeks’ time.
The government provoked union outrage when it resorted to a constitutional device to force its watered-down labour reforms through parliament without a vote.
The government says the reforms, which make it easier for companies to hire and fire staff, are needed to bring down unemployment.
French labour reform bill – main points
- The 35-hour week remains in place, but as an average. Firms can negotiate with local trade unions on more or fewer hours from week to week, up to a maximum of 46 hours
- Firms are given greater freedom to reduce pay
- The law eases conditions for laying off workers, strongly regulated in France. It is hoped companies will take on more people if they know they can shed jobs in case of a downturn
- Employers given more leeway to negotiate holidays and special leave, such as maternity or for getting married. These are currently also heavily regulated