Arrivals and departures were cancelled at London City Airport where forecasters said visibility was down to below 100 metres.
Flights have also reportedly been cancelled at Heathrow, Gatwick and Manchester airports.
Passengers at London City Airport were urged to get in touch with airlines directly to find out the latest news on their flights.
The airport, at the Royal Docks in Newham, east London, said on its website: "Due to low visibility at London City Airport this morning flights are experiencing disruptions. Passengers are advised to contact their airlines for more information."
At least 10 departures have been cancelled, among them the 7am flight to Edinburgh, the 7.30am to Zurich, the 7.35am to Rotterdam and the 7.50am to Amsterdam.
Heathrow airport reported "minor delays" on a "few" in-bound flights, but said there were no cancellations or delays to flights from the airport.
The fog is expected to clear in the area by 9am.
Gatwick said it was operating normally with no delays or cancellations.
A spokesman said: "We have had one flight re-diverted to us from London City airport, but otherwise everything is operating 100% as normal."
London City Airport wrote on its twitter page: "Due to low visibility flights to/from LCY are experiencing disruptions. Passengers should contact their airlines."
Chris Burton, from forecasters MeteoGroup, said: "The fog is quite extensive across much of England and Wales this morning, and quite a few places will be waking up to quite foggy conditions.
"But as we go through the morning the fog and cloud should break up, and by afternoon we should see some good spells of sunshine."