Dozens of UK flights at two major British airports were cancelled on Monday as foggy weather hit much of England.
Heathrow — which was one of the airports affected — said poor visibility meant the rate at which planes could take off and land was reduced.
The airline apologised to customers and said it was "doing everything we can to get them on their way as quickly as possible".
It urged customers to check the status of their flights online before departing.
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.”
The UK experienced unseasonably warm weather over the weekend, which was replaced with fog in the south-east of England.
In an update, the Met Office said the UK could expect unsettled weather over the week.
Met Office spokeswoman Nicola Maxey said there was a chance of thunder on Wednesday in the south and west of England.
She said the weather will remain unsettled as the week progresses, although there will be some drier spells.
“Temperatures are expected to drop this week back towards what we would expect for this time of year", she said.