Freezing fog is causing extensive flight delays today, particularly at London Heathrow, Gatwick and City airports, and also at Amsterdam and Southampton.
The warning was issued this morning by Eurocontrol, the European Organisation for the Safety of Air Navigation.
British Airways and other airlines have cancelled flights due to thick fog across parts of southern England.
Around 100 flights have been cancelled at London Heathrow and 37 at London City as air traffic controllers impose restrictions on schedules in poor visibility.
BA warned other flights are likely to be delayed throughout the morning and said it was also expecting the fog to affect some flights at London Gatwick.
National Air Traffic Services (NATS) has asked airlines to reduce their schedules to allow greater space between landing aircraft in fog.
London Stansted, Liverpool and other UK airports are also advising passengers to check with their airline before travelling and Amsterdam Schiphol warned of flight delays and cancellations.
