(CNN) -- French air traffic controllers are scheduled to strike Wednesday, affecting flights out of the two main Paris airports, the French civil aviation authority said.
"Due to a social movement, Civil Aviation Management (DGAC) has asked airlines to reduce their flights on Wednesday, July 21," the agency said in a statement.
The country's weather service Meteo France is forecasting stormy weather for the same day, which could disrupt air traffic further.
The strike is starting on Tuesday evening with the cancellation of about 10 EasyJet flights at Orly airport, CNN affiliate France 2 said.
French air traffic control has asked airlines to cancel 20 percent of flights to and from Roissy Charles-de-Gaulle and 50 percent of flights in and out of Paris Orly.
The agency recommends that passengers contact their airline before going to the airport Wednesday.
Controllers are striking over plans to unify European air traffic control.
The CGT, one of the unions representing air traffic controllers, said on its website that five unions are calling for workers to strike to show their opposition to the European fusion project.
The unions predict "heavy social consequences" if the project goes ahead, including the "dismantlement" of France's national air traffic control, and "the exit of all or part of the services and public sector workforce."
France is due to join Germany, Switzerland, Belgium, the Netherlands and Luxembourg to form what is being called Functional Airspace Block Europe Central (FABEC).
FABEC is a cornerstone of the Single European Sky project, which advocates say will reduce fuel use and flight times by making it possible for planes to fly more direct routes across the continent.
The FABEC area covers 1,713,442 square km (661,560 sq miles) and is characterized by closely interlaced civil and military traffic routes, its website says.
Most of the large European airports and major civil and military airways are located in the FABEC area.
CNN's Pierre Meilhan and Lianne Turner contributed to this report.