Heathrow Airport has cancelled one in ten flights today due to low visibility.
It took the decision to cancel services last night, saying that not doing so would cause significant disruption to passengers and flights.
But it warned that it was possible poor weather conditions at other European airports would increase the number of flight cancellations throughout today and urged passengers to check the status of their flight with their airline before travelling to the airport.
"When there is low visibility, more space has to be left between aircraft. Many airports have plenty of spare runway capacity so aircraft can be spaced out more during low visibility without causing delays and cancellations. Because Heathrow operates at almost full capacity, there is simply no room to reschedule the delayed flights," it said.
British Airways said passengers should only go to the airport if their flight is operating. Shortly after midnight it posted this notice on its website: "We are doing everything we can to help you if you have been disrupted and we fully apologise for the inconvenience caused to your travel plans.
"Like other airlines at Heathrow we have complied with a request to reduce our schedule by 10% on Monday."
BA is giving passengers due to travel today and tomorrow the option to rebook for flights departing between January 24 and February 12 at no extra costs.
Virgin Atlantic said it planned to continue operating as normal today, but advised passengers to check their flight was operating before travelling to the airport.
Other London airports were all open this morning but warned of possible delays due to the weather.
Heathrow said cancelling flights in advance introduces space into the schedule and aims to reduce disruption for passengers by:
Monday, January 21, 2013