Air passengers will face a second day of disruption as another raft of flights are cancelled due to dense fog.
Around 45 flights, 10% of departures from Heathrow airport, have been "pre-cancelled" as a yellow weather warning for fog covers much of England and east Wales.
After long delays on Sunday some passengers landed at Britain's busiest airport in the early hours of the morning, but many hoping to take off today will be disappointed.
A spokeswoman for Heathrow 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."
Passengers are advised to check with their airline for updates.
Those hoping to travel through Gatwick have also been advised to check ahead for any disruption.
Some passengers told of their relief on reaching London after delays on Sunday, which also saw around 50 flights cancelled at Heathrow.
Jennifer Johnston: "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 BritishAirways 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."
Visibility could be reduced to less than 100 yards, it added.