Both Heathrow and London City have told passengers to expect delays and cancellations this morning as reduced visibility causes problems, while Gatwick has said it is monitoring weather conditions closely.
Around 100 flights of 1,300 have been cancelled at Heathrow, while London City has cancelled about 15 and Gatwick one flight. Stansted hasn't yet cancelled any.
A Heathrow spokesperson said: "Foggy weather across the South East has reduced visibility at Heathrow. As a result, some passengers may experience disruption to their journeys today. Passengers should check their flight status with their airline before coming to the airport. Heathrow's top priority is the safety of passengers and we apologise to those whose travel has been affected."
The airport has warned before that as it's operating at capacity, the effect of fog on flights can be more noticeable there than at other airports, as it doesn't have the spare runway capacity to space out aircrafts without causing delays and cancellations.
Foggy weather is expected to cause some disruption to flights at Heathrow today. Please check with your airline prior to travelling.— Heathrow Airport (@HeathrowAirport) January 23, 2017
Passengers please note delays & cancellations are expected this morning as a result of fog. Please contact airline for info on your flight.— London City Airport (@LondonCityAir) January 23, 2017
Sub-zero temperatures are being felt across the country, and the Met Office has issued a yellow fog warning, telling people to be aware, for the majority of southern England until 10.30am.
Forecasters have warned of difficult driving conditions and delays at some airports.