By Richard Lough

PARIS (Reuters) -French air traffic controllers began a two-day strike on Thursday to protest over staff shortages and ageing equipment, leading to hundreds of flight cancellations just as the summer season gets under way.

France's civil aviation agency DGAC told airlines to revise their schedules, including at Paris' Roissy Charles de Gaulle airport - one of Europe's busiest hubs - forcing the carriers to cancel flights. 

Air France, France's largest airline, said it had adapted its flight schedule, without giving details, but that it was maintaining its full long-haul flight schedule. IAG-owned British Airways was using larger aircraft to mitigate disruption.

Ryanair said it had been forced to cancel 170 flights affecting over 30,000 passengers on Thursday and Friday.

"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."

Easyjet said it would be cancelling 274 flights over Thursday and Friday.

The strike coincided with the start of the European summer holidays, one of the busiest travel periods of the year.

France's second-largest air traffic controllers' union, UNSA-ICNA, said its members were striking over persistent understaffing, outdated equipment and a toxic management culture. Another union, USAC-CGT, said the DGAC had failed to comprehend the frustration felt by controllers.

"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. 

"The systems are on their last legs, and the (air traffic control) agency is constantly asking more of its staff to compensate for its difficulties," it added.

The DGAC did not immediately respond to a request for comment on the trade unions' concerns.

Their complaints echo grievances expressed by air traffic controllers in the United States over outdated infrastructure, dramatic staffing shortfalls and failing technology.

French Transport minister Philippe Tabarot called the unions' demands unacceptable.

The DGAC asked airlines to cut one in four flights in and out of Paris airports and almost half of flights out of the capital on Friday. Elsewhere, airlines were asked to reduce flights by 30%-50%, with the south particularly hard hit.

"Despite these preventative measures, disturbances and significant delays are to be expected at all French airports," the agency said, urging passengers to change their flights if they were able to.

Luxair Luxembourg Airlines warned that "additional delays and schedule changes are possible across other destinations, as air traffic rerouting and capacity constraints may cause knock-on effects throughout the network."

Ryanair's O'Leary urged the European Commission, the European Union's executive arm, to reform EU air traffic control services to ensure adequate staffing at peak periods and to protect overflights - those that pass over a country or region without landing there - during national strikes.

(Reporting by Makini Brice, Richard Lough and Sudip Kar-Gupta in Paris, Sarah Young in London; Editing by Mark Potter)