A strike by French air traffic controllers seeking better working conditions brought chaos at the height of Europe’s summer travel season after around 40% of flights to and from Paris were canceled Friday.

Disruptions started hitting French airports Thursday. These intensified Friday as the national civil aviation authority asked airlines to cancel 40% of flights at Charles de Gaulle, Orly and Beauvais airports serving Paris, half of flights in Nice and 30% of flights in Marseille, Lyon and some other cities.

Despite the preventive cancellations, the authority warned in a statement that “disruptions and long delays are to be expected at all French airports.”

Transport Minister Philippe Tabarot called the unions’ decision “unacceptable.” — Associated Press