Nearly 2,000 flights have been cancelled across Europe due to an ongoing strike by French air traffic controllers.
The industrial action comes as workers protest a plan to simplify Europe's patchwork airspace and open up more air traffic duties to private enterprise.
French officials confirmed that some 1,800 flights were cancelled for a second day in the country.
The cancellations totalled about half the normal number of flights, according to the French authority that tracks airport conditions.
Ryanair cancelled 293 flights, a third which were to and from the UK, while EasyJet cancelled 200, just under half to and from the UK.
The British Airways website showed 16 flights were grounded on Wednesday morning.
An easyJet spokesman said: "We are notifying passengers by email, text message and on the website. We recommend passengers visit www.easyJet.com to check the status of their flight.
"Passengers on cancelled flights are able to receive a refund or transfer to another flight."
Flights from London's Heathrow, Gatwick, Manchester and Glasgow airports have seen flights stopped.
BA said in a statement: "French Air Traffic Control have amended their announced industrial action dates and will now only strike on Tuesday June 11 and Wednesday June 12.
"As a result BA intends to operate a full schedule for Thursday June 13.
"Due to airspace restrictions in and around France, there may also be delays for services to and from Spain and North Africa.
Airline passengers travelling elsewhere in Europe have been warned to expect delays of up to four hours.
Those booked on flights today were urged to check with their airlines to ensure services were running on the second day of industrial action.
The umbrella union for air traffic controllers said 11 countries would take part, but the significant industrial action appeared limited to France.
French airports including Charles de Gaulle, Orly, Lyon, Nice, Marseille, Toulouse and Bordeaux are all said to have been affected by the strikes which were called across Europe by the European Transport Workers' Federation (ETF) in April.
The group said action would involve protests and walkouts by union members across the European Union to "stop a never ending process of liberalisation, deregulation and cost cutting".
Air traffic controllers say they fear threats to passenger safety and to their jobs and claim the EU is yielding to industry pressure to cut costs.
Travel group Abta criticised the industrial action and said that passengers whose flights were cancelled would be entitled to a refund or a replacement flight and should contact their airline.