At least 95 flights have been cancelled after thick fog enveloped Heathrow Airport.
A spokeswoman said: "Due to low visibility caused by heavy fog, the airport is currently experiencing delays to its service."
The fog has also affected several flights leaving from Cardiff Airport, many of which were delayed while some were diverted to other airports.
Passengers have been asked to check their flight status.
Most of the flights cancelled by Heathrow Airport were to or from short-haul destinations.
On average about 1,300 flights fly from and into the west London airport a day.