Thick fog is disrupting flights at UK airports for a second day.
Around 45 flights - equivalent to 10% of departures - have been cancelled from Heathrow Airport as a yellow weather warning for fog covers much of England and east Wales.
London City Airport reported that a "considerable number" of flights have been cancelled, while Gatwick also expected to have cancellations due to poor visibility caused by the fog.
Flights from other UK airports to the capital and some European cities were also disrupted by the fog.
Passengers are being advised to check the status of their flight with the airline before travelling to the airport.
After long delays on Sunday some travellers landed at Heathrow in the early hours of the morning, but many hoping to take off today will be disappointed.
A spokeswoman for the airport said: "We've got extra passenger ambassadors in the terminals to provide assistance for those travelling today.
"Once again we would like to apologise for the disruption."
Some passengers told of their relief on reaching London after delays caused by fog on Sunday, which also saw around 50 flights cancelled at Heathrow.
Jennifer Johnston wrote: "Safely landed at Heathrow. British Airways I forgive you for our 5 hr delay. The fog is real and thick!! "
Tom wrote: "A 2 hour delay but would like to thank British Airways for getting us back into Heathrow safely tonight despite the dense, lingering fog."
The Met Office weather warning is valid until midday, with fog expected to slowly clear throughout the morning.
The forecaster said the fog would become widespread and dense overnight into today and warned: "Travel may be impacted by poor visibility, with some minor disruption expected."
Sky News presenter Eamonn Holmes was one of those stranded by the fog.
He was unable to make it to London in time for the Sunrise television news programme at 6am.
He tweeted on Sunday: "Not getting out of Belfast" after flights were cancelled and added that he had been unable to join his colleague on the show.
Early flights to Manchester, London City and Heathrow airports were cancelled from Belfast City Airport.
Visibility could be reduced to less than 100 yards, the forecaster added.
Motorists have been advised to take care on the roads amid the thick fog.
Highways England urged people to allow extra time.
The agency said on its website: "Road users will need plenty of extra time to do their usual commute, especially as the fog is expected to lift very slowly during the morning."
West Midlands Ambulance Service tweeted: "Weather update. It's very foggy!!! Drive slowly, use fog lights or better yet stay inside."