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Workers at French nuclear power stations are due to down tools 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.
Petrol is already in short supply after days of strikes, and nuclear power stations were reducing operations ahead of Thursday's action.
PM Manuel Valls has indicated he could modify the labour laws.
He told French TV that "there may still be changes, improvements" but the reforms would not be withdrawn.
The strikes have put huge pressure on the government, which is preparing to host the Euro 2016 football championships in two weeks time.
There have been long queues at petrol stations as strikes and blockades disrupting six of France's eight oil refineries affected supplies.
Transport Minister Alain Vidalies said 40% of petrol stations around Paris were struggling to get fuel.
France has nearly four months of fuel reserves, analysts say.
President Francois Hollande told ministers on Wednesday that "everything will be done to ensure the French people and the economy is supplied".
Clashes
Nuclear power provides about 75% of the country's electricity.
Grid operator RTE said French nuclear power capacity was being cut by at least four gigawatts - equivalent to 6% of the country's total production capacity - Reuters news agency reports.
Queue at petrol station in Le Havre, north-western France, 25 May 2016Image copyrightEPA
Image caption
Long queues are forming at petrol stations across France
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.
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 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.
Grey line
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
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