Dense fog has forced at least 280 flight cancellations at London's Heathrow Airport, causing disruption for thousands of air travellers.
Further cancellations are likely on Thursday as staff deal with the backlog and more fog, a spokesman warned.
London City Airport was shrouded in fog through the day, with every flight on Wednesday morning cancelled or delayed.
At Southampton Airport, the fog had cleared by midday but 16 flights were cancelled in the morning.
A Heathrow spokeswoman said that by late evening, passengers were experiencing delays of between 30 minutes to an hour.
She said most passengers, whose flights were grounded on Wednesday, had been rebooked onto flights scheduled to depart later on Thursday.
The cancellations - which included flights to Berlin, Paris, Belfast and Dublin - were mostly short-haul journeys, she said.
Three inbound flights were diverted to Stansted, which was unaffected by the fog. Luton and Gatwick reported no cancellations and only minor delays.
A London City Airport spokeswoman said the fog had stayed for most of the day and advised passengers to check with their airlines before travelling to the airport.
She said she was unable to provide figures for the number of flight cancellations, but the departures board showed at least 30 cancellations on Wednesday afternoon.
On Tuesday night four aircraft had to be diverted from Southampton airport to Bournemouth.
A spokeswoman for Heathrow said on average the airport handles about 1,300 flights a day.
The airport has had to cancel flights because of an air traffic control requirement for more space on the schedule between planes during periods of decreased visibility, she added.
Stephen Main, who was due to fly to New York from Glasgow via London Heathrow, said he would now be delayed by almost nine hours.
"We had plans for a business meeting in the afternoon but this puts the whole trip back."
A spokeswoman for Southampton Airport said they were still experiencing some delays during the afternoon even though the fog had cleared.
Delays were reported early in the morning on the crossing between Hampshire and the Isle of Wight, Wightlink Ferries said.
And operations at Southampton Container Port were suspended due to the fog.
BBC weather forecasters said the morning's fog in London was 150m deep in places.
More dense fog is likely overnight in south-east England but the breeze is expected to move it away quickly, forecasters say.