Flights have been delayed and drivers are being warned about visibility as the UK sees more heavy fog.
It comes after 20,000 people were affected by airport disruption on Friday, according to travel journalist Simon Calder.
National Air Traffic Services (NATS) said: "As widespread fog continues to affect some UK airports today, temporary air traffic restrictions remain in place at locations with low visibility.
"Restrictions of this sort are only ever applied to maintain safety... Our teams are working closely with the airports and airlines to minimise disruption."
A spokesperson for Gatwick Airport said: "Temporary air traffic restrictions have been put in place due to fog causing poor visibility. Some flights may be delayed throughout the day."
NATS and the airport urged passengers to check their flight's status before travelling.
Kiera Quayle and her husband are stuck on the Isle of Mann after visiting his family for Christmas.
On Friday night, their Easyjet flight back was delayed and then cancelled due to Gatwick's fog and their new flight doesn't leave until 5pm on Sunday.
"We're lucky that we have family we can stay with, but it has been difficult organising travel and where we are staying," she told Sky News.
"Luckily for us our neighbour has agreed to look after our cat for a few extra days, and we can swallow the extra car park charge at Gatwick," she said.
At Gatwick on Saturday, 32 flight arrivals have been delayed between 4.15pm and 6.15pm and 42 departing flights have been delayed.
At Heathrow, 7 flight departures scheduled to leave between 4.15pm and 6.15pm have been delayed, and one has been cancelled.
The thick fog will continue on Saturday according to the Met Office, with some areas seeing visibility reduced to just 100 metres.
Drivers are being warned to leave extra braking distance in areas with poor visibility while travellers should allow "a little bit longer" for their journeys, said the Met Office.
Get the weather forecast where you are
The worst of the fog is forecast to hit the South East and central England, while East Anglia and parts of South Wales will also be affected.
Forecasters have not issued any fog weather warnings yet but the Met Office said it will monitor the situation across the weekend.
Met Office meteorologist Liam Eslick said: "It's that time of year when people are travelling around the country a lot and there are a lot of people on the roads.
"There is a lot of fog covering much of England, mainly the South East and central England, but the rest of the country is seeing quite a bit of thick fog too.
"It will be pretty murky on Saturday morning and there will still be fog patches that will take a little longer to clear."
Sunday is expected to be Gatwick's busiest day during the holiday period, with 769 flights planned.
Meanwhile, there is a yellow warning in place for heavy rain in parts of Scotland for Monday and New Year's Eve on Tuesday, leaving the start of 2025 "distinctly unsettled".
Edinburgh's Hogmanay celebrations could face "significant disruption" from rain, with organisers warning audiences to "prepare for all weather, wrap up warm and double-check with your travel operator for the latest updates".
The yellow warning could see as much as 140mm (5.5in) of rain in places, as other parts of the UK will likely see rain, wind and possibly even snow later on next week.
The Met Office added that "more warnings are likely to be issued".
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A spokesperson for the UK's main air traffic control provider Nats said it had implemented "temporary air traffic restrictions" at several airports on Friday due to the fog.
While the fog likely won't remain past the weekend, people should not hold out hope for sustained sunshine with wet and windy conditions expected for much of the country, the Met Office said.
(c) Sky News 2024: UK weather: Dozens of flights delayed as major UK airport issues fog warning