Aer Lingus has cancelled two flights to Heathrow due to severe fog. Flight EI-154 and flight EI 155 are the affected services.
Bad weather has caused widespread disruption across the UK as schools return from the half-term break.
Some flights are being cancelled or diverted due to the weather, and passengers are advised to check the status of their flights before travelling to airports.
Airports affected include London Heathrow, Gatwick, Birmingham, Leeds Bradford and Liverpool.
Heathrow cancelled more than 40 - around 10% - of its flights on Sunday.
A British Airways flight from Sao Paulo to London was diverted to Glasgow because of thick fog, and an Air India plane was diverted to Paris.
The Met Office has issued a yellow weather warning for most of England and parts of Wales tonight, and drivers are being urged to slow down because of reduced visibility.
Dense fog is expected to remain on Monday morning, causing traffic delays as pupils return to school after the week-long holiday.
Sky News weather presenter Isobel Lang said: "Fog has formed widely this morning with visibility below 100 metres in many areas.
"It will cause delays at some of the major airports, and make driving slow and difficult at times."
She said bonfires and firework displays will also add to the gloomy conditions this week as people celebrate Guy Fawkes Night.
"Smoke particles are hygroscopic - they attract water molecules from the atmosphere - which encourages fog to form at lower humidity levels than in less polluted air," Lang added.
