Heathrow airport authorities said around 45 flights, about 10 percent of departures, had been cancelled, while London City airport has been closed to all in and outbound flights until at least 11:00 (1630 IST).
Other affected airports include Manchester, Leeds Bradford, Glasgow, Belfast and Cardiff.
"We've got extra passenger ambassadors in the terminals to provide assistance for those travelling today. Once again we would like to apologise for the disruption," a Heathrow Airport spokesperson said.
Easyjet said it was suffering "major network disruption", with a large number of aircraft and crew out of place and numerous delays, diversions and cancellations.
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A Met Office yellow weather warning for fog, covering much of England and east Wales, is in force and motorists were advised to take extra care on the UK's roads today morning when many schools return from a week-long half-term break.
Yesterday, a driver died and a five-year-old girl was found injured in a crashed car alongside her dead father in Staffordshire county of West Midlands.
Highways England said on its website: "Road users will need plenty of extra time to do their usual commute, especially as the fog is expected to lift very slowly during the morning.
"Forecasters said the fog may linger into the afternoon for some eastern parts of England and southern Scotland.
Yesterday, an Air India flight had to be diverted to Paris as 50 flights were either cancelled or diverted due to thick fog at Heathrow airport.
Britain's busiest airport said the weather had forced the cancellation and advised passengers to check their flight status before heading to the airport.