A total of 18 flights - nine arrivals and nine departures - at Heathrow were cancelled on Wednesday morning.
At 11:00 GMT, an airport spokesman said: "The fog is clearing rapidly and there are no delays that I'm aware of."
Flights to and from Heathrow and London City airports were also disrupted on Sunday and Monday due to fog.
A Heathrow Airport spokesman said cancellations were due to "reduced capacity on the runways caused by the thick fog".
"It is hoped that few, or no, further cancellations will be necessary as the fog is expected to clear," he added.
"Passengers should check with their airline that their flight is going ahead."
At London City Airport, nine flights - four arrivals and five departures - were cancelled, while further flights were diverted or delayed.
An airport spokeswoman said: "We are asking customers to contact their airlines and are constantly updating our website to keep customers informed."