Strike action by French air traffic controllers on Thursday has led to the cancellation of hundreds of flights, leaving tens of thousands of passengers in the lurch at the start of the summer season — one of the busiest times of the year for travel.

France's civil aviation authority DGAC said airlines needed to revise their schedules, including at Paris' Roissy Charles de Gaulle airport — one of Europe's busiest travel hubs.

A little after 10 am (0800 UTC/GMT) on Thursday, AFP news agency reported that flights were experiencing significant delays, including an average of 1.5 hours for arrivals and 1 hour for departures in Nice, France's third-largest airport.

Ryanair CEO slams strike action

Europe's largest airline Ryanair said that it had been forced to cancel 170 flights over Thursday and Friday, impacting over 30,000 passengers.

"Once again European families are held to ransom by French Air Traffic Controllers going on strike," Ryanair CEO Michael O'Leary said.

"It makes no sense and is abundantly unfair on EU passengers and families going on holidays," O'Leary said. 

Another popular budget airline, Easyjet, said it would be cancelling 274 flights over the same period.

The Airlines for Europe association, which includes Ryanair, Air France-KLM, Lufthansa, British Airways and EasyJet, described the action as "intolerable".

Why are air traffic controllers striking

Air traffic controllers' union, UNSA-ICNA, said members were taking action over understaffing, outdated equipment and a toxic management culture.

"The DGAC is failing to modernise the tools that are essential to air traffic controllers, even though it continues to promise that all necessary resources are being made available," UNSA-ICNA said in a statement.

 France's transport minister Philippe Tabarot said the unions' demands are unacceptable.

"The demands made by minority unions are unacceptable, as is the decision to hold this strike at the start of the holiday season," Tabarot said on Wednesday.

Edited by: Elizabeth Schumacher