Hundreds of flights to and from France have been cancelled as air traffic controllers begin industrial action.
The SNCTA union - France's largest - called the two-day strike in a dispute over working conditions.The DGAC civil aviation authority has asked airlines to cut their scheduled flights by 40%.
Short-haul flights have been the worst affected but France's largest airline, Air France, said long-haul flights were still operating.
The airline added that 60% of medium-haul flights from and to Paris' main airport, Charles de Gaulle, would still operate, but that it would ground two out of three flights at Paris' second-largest airport, Orly.
"Disruption is expected over the whole country," the DGAC said in a statement.
Travellers have been advised to contact their airline.
France's civil aviation agency said part of the dispute involves plans to raise the retirement age for controllers from 67 to 69 years.
Further strikes are planned for 16-18 April and 29 April-2 May, coinciding with spring school holidays in France.
Low-cost airline Ryanair says it has had to cancel more than 250 flights, with further cancellations likely.
'Grossly unfair'
"We sincerely apologise to all customers affected by this unwarranted strike action and we call on the EU and French authorities to take measures to prevent any further disruption," said Ryanair in a statement."It's grossly unfair that thousands of European travellers will once again have their travel plans disrupted by the selfish actions of a tiny number of French ATC [air traffic control] workers."
Travellers booked on short and medium-haul Air France flights on Wednesday or Thursday can choose to travel between 10 and 15 April instead, it said.