More than half of Heathrow's departing flights were showing as delayed, according to Flightradar24.
The same airports also saw disruption on Friday due to the foggy conditions.
"As widespread fog continues to affect some UK airports today, temporary air traffic restrictions remain in place at locations with low visibility," the air traffic control authority NATS said.
It said the restrictions were to maintain safety and that its teams were working closely with the airports and airlines to minimize disruption.
In some areas, thick fog patches could reduce visibility down to just 100 meters (328 feet), Britain's weather agency, the Met Office, said.