Around 100 flights have been affected at Heathrow, and a further 37 flights have been cancelled at London City.
Gatwick has also warned that delays are likely.
Heathrow has tweeted a warning to passengers that they may experience disruption.
https://twitter.com/HeathrowAirport/status/823424727389794304
Passengers have been advised to check their flight status with their airlines.
Heathrow reportedly added that the effect of fog on flights was more noticeable than at most other airports due to the fact that it operates at 98% capacity.
London City Airport has said that 20 departure flights and 17 arrivals have been cancelled, the BBC said.
Luton Airport has not cancel any flights, although a number are understood to have been delayed.
Southampton has not yet cancelled any flights, but was monitoring the situation.
