Hundreds of flights are set to be cancelled, and many more delayed, due to the first French air-traffic control strike of the summer.
Controllers and engineers working for DSNA, the national air-navigation provider, will stop work from 7pm local time on Wednesday 8 May until 6am on Friday.
The 35-hour stoppage is part of a wider national protest by public servants in France against President Macron’s plans to reform their working conditions.
Because so many aircraft normally overfly France, widespread delays and cancellations are likely.
The French authorities have stipulated a minimum service level of 50 per cent of overflights – those which do not land or take off in France – and some flights will be able to avoid French airspace.
Switzerland is opening up additional airspace sectors to take some of the strain, while extra traffic is expected on the "Tango" routes west of French airspace.
But taking circuitous routes will cause delays to build as the strike progresses.
Paris Charles de Gaulle, the nation’s leading hub, will not itself have cancellations directly caused by strikes at the airport, because air-traffic controllers are expected to work near-normally.
But operations to and from “CDG” it will be impacted by the reduction in flights handled by Area Control Centres in France.
France is divided into five airspace areas: Bordeaux, Brest, Paris, Reims and Marseille.
The biggest impact is likely to be in the Marseille Area Control Centre, which covers the southeast quadrant of France plus a large patch of the Mediterranean.
During a conference call with Eurocontrol, DSNA representatives said that four French airports – Paris Orly, Lyon, Marseille and Toulouse – would be particularly affected, because of the high propensity for controllers at these airports to strike.
Airlines have been instructed to reduce the number of scheduled flights to and from these airports by 30 per cent.
Nantes will also be hit, and Perpignan and Pau airports will be closing at night.
One airline, easyJet, has already cancelled 54 departures, saying: “Any customers affected by cancellation are eligible for a refund or a free of charge transfer to a new flight.
“Customers on affected flights will be contacted directly via SMS and email address via the details provided at the time of booking.”
Around 60 per cent of easyJet flights normally go through French airspace. The carrier is warning that other flights will experience delays.
British Airways has cancelled a total of 36 flights, including six flights to and from Paris, four between Heathrow and Nice, and single round-trips from Heathrow to Lyon and Marseille.
In addition BA has grounded six flights between Heathrow and Barcelona, and a round-trip to Geneva and Basel.
Ryanair has also cancelled some flights pre-emptively, with passengers informed on Tuesday afternoon and offered refunds or re-booking.
The Irish airline is telling passengers: “If your flight is delayed, you will be sent an email and SMS on the day with the updated scheduled times of departure.
“We apologise for any inconvenience caused by these strikes, we will do everything we can to minimise the disruption, which is completely outside of our control.”
During third-party strikes such as this, airlines are obliged to provide meals, accommodation and alternative flights as necessary to disrupted passengers. But they are not liable to pay compensation.