Dozens of UK flights were cancelled on Monday morning due to fog.
Heathrow – which was one of the airports affected – said poor visibility meant the rate at which planes could take off and land was reduced.
British Airways cancelled 15 departures from Heathrow, which were all to domestic and short-haul destinations.
The airline said in a statement: “Like other airlines, we are experiencing some disruption to our London Heathrow services this morning due to fog.
“We apologise to customers and are doing everything we can to get them on their way as quickly as possible.
“We advise customers to check ba.com for the latest flight information.”
Another airport, London City, began cancelling flights on Sunday afternoon, with 23 departures scrapped.
Disruption from fog continued into Monday, with 30 departing flights cancelled.
Anna Bowles, head of consumer at regulator the Civil Aviation Authority, said: “If a flight is delayed due to adverse weather conditions, airlines have a duty of care to look after their passengers, which can include providing food and drink, as well as accommodation if people are delayed overnight.
“Although compensation is not usually payable for delays caused by circumstances beyond an airline’s control, we expect them to proactively provide passengers with information about their rights when flights are disrupted.
“We have guidance on cancellations and flight disruption published on our website and expect airlines to follow this.”