Heathrow has cancelled 45 flights, the BBC reported, with delays at London City and Glasgow airports.
"We are currently experiencing flight disruptions due to fog. As a result your flight may be disrupted," easyJet said on its website.
The airline added: "We would like to apologise for any inconvenience and thank you for your patience."
British Airways said: “Heavy fog across southern England is continuing to cause flight disruption at many UK airports, including London Heathrow, London Gatwick and London City, on Monday 2 November.
“At London Heathrow, the Airport Authorities and Air Traffic Control have again requested airlines to reduce their schedules on Monday to maintain safety.
“Please do not come to the airport unless you have a confirmed booking on a flight that is operating.”
