Tens of thousands of passengers have been hit by a French air traffic control strike that has cancelled flights in France on Thursday and Friday and had a knock-on effect elsewhere in Europe.

Budget airline Ryanair said it had been forced to cancel more than 170 flights and the holiday plans of more than 30,000 passengers had been disrupted.

Two French unions were staging the two-day strike over working conditions, leading to a quarter of flights being cancelled at the main airports in Paris and half of flights at Nice airport.

French Transport Minister Philippe Tabarot condemned as unacceptable both the unions’ demands and their decision to go on strike at such an important time for people going on holiday.

Ryanair said the strike had affected not only its flights to and from France, but also aircraft flying over French airspace to destinations including the UK, Ireland, Spain and Greece.

Its chief executive, Michael O’Leary, accused the air traffic controllers of “holding European families to ransom”.

“It makes no sense and is abundantly unfair on EU passengers going on holidays,” he complained, calling on European Commission President Ursula von der Leyen to take “urgent action” to ensure minimum service levels during strikes and to protect flights going through French airspace from domestic industrial action.

France’s civil aviation authority, DGAC, asked airlines to reduce flight schedules at several airports across the country.

Disruption is expected to worsen at French airports on Friday, with 40% fewer flights from the Paris airports at Charles de Gaulle, Orla and Beauvais.

The strike was called by the UNSA-ICNA union, which cited staffing shortages, management issues and the planned introduction of a controversial clock-in system for controllers among its concerns. Talks with the DGAC earlier this week failed to resolve the dispute.

Airlines for Europe (A4E), the continent’s leading aviation body, described the strike as “intolerable”, warning that it would severely disrupt travel plans at the height of the holiday season.

EasyJet, another low-cost carrier affected by the disruption, expressed “deep disappointment” and urged a resolution.

Ryanair said on Wednesday it also had been hit by the recent conflict in the Middle East and had cancelled more than 800 flights last month.

Despite the cancellations, the airline said it still operated more than 109,000 flights in June, indicating that fewer than 1% of flights were affected.