About 5,344 megawatts (MW) of coal and fuel-oil-fired or so-called thermal generation has been shut, out of the country's total electricity production capacity of 115,000 MW.
"The strike is continuing today and there are cuts in production in both nuclear and thermal plants," the CGT spokesman said.
The CGT has called for weeklong industrial action, including disruption in power supplies to public and EDF buildings. Action by regional federations to reduce production began on Tuesday night but had little impact on supplies in the French power market.
"Snet workers joined in at midnight last night and all four plants' production is expected to remain at zero all day," the union spokesman said.
A spokesman from Snet, 19-percent owned by EDF confirmed that all four of its coal-fired plants with 2,474 MW of coal-fired capacity, were shut.
EDF officials were not available to comment.
The union said EDF workers on Thursday morning shut down the 1,170 MW fuel-oil fired Porcheville plant.
The strike had on Wednesday already shut the 250 MW Montereau coal-fired power station and the 1,450 MW Le Havre coal plant.
The French unions claim the government's plans to change by the end of the year the utility's legal status -- the precursor to its partial privatisation plans -- will affect services to the public.
In their last strike on January 20, French energy workers cut 6,800 MW, or seven percent of EDF's output capacity.
French unions have threatened in previous strikes to slash output but have never shut down more than 12 percent of EDF's production capacity, made up of 63,000 megawatts of nuclear, 16,500 MW of thermal and 20,400 MW of hydropower.
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