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He told French TV that "there may still be changes, improvements" but the reforms would not be withdrawn.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, foreign media reports.