Heavy fog over the capital and elsewhere across England and Europe has caused airports to warn passengers of potential delays and cancellations.
"Heavy fog across England and Europe is causing disruption to flights leading to delays and cancellations," said Heathrow and Gatwick in messages to customers on their website.
Customers are being advised to check flight details with their airlines before they leave for the airport
British Airways said some if its flights had been cancelled and that there would be fewer flights with Air Traffic Control increasing time for landing in thick fog.
EasyJet said it was experiencing "some disruption" to its network.
The weather disruption comes ahead of misery for Christmas travellers, with strike action by Virgin Atlantic pilots, British Airways cabin crew and Swissport baggage handlers.