Severe fog has forced the cancellation of dozens of flights scheduled to take-off and land at Britain's airports on Wednesday with more disruption expected later in the day.
Flights to Warsaw, Nice, Stockholm, Aberdeen and several German cities were among 20 cancelled by London's Heathrow.
"It's better to cancel flights when there's fog because in fog you can't land as many planes per hour because you have to leave more space between the planes," said a spokesman for the airport.
The smaller London City Airport also reported disruption.
A message on the airport's Twitter page said: "Visibility has not improved, flight disruptions are present and expected to continue until further notice. Please call your airline for info."
Britain is in the middle of a cold snap with temperatures hitting minus 10C (14F) in some parts of the country.
"The fog is currently pretty widespread," explained Julian Mayes, forecaster for MeteoGroup.
"It's currently located over central England as a large area of low cloud, which is descending down as fog.
"Visibility is poor and the areas affected include the Midlands, the eastern side of Wales, most of East Anglia, most of the area just to the north of London, Bristol and parts of central southern England," he added.