Flights have been cancelled at London airports as thick fog swathes parts of south and south east England on Monday.
Both airports warned passengers to check with their airline before travelling.
One flight was cancelled at Gatwick.
The Met Office had yellow ‘be aware’ warnings in place for the east of England, south west England and London and the south east.
The warnings are in place until 10.30am but freezing temperatures and low winds could see it persist into the afternoon, according to forecaster Mark Wilson.
The chief forecaster’s comments read: “As the cold airmass across much of the south of England moistens up, there will be a much greater risk of fog than on recent nights.
“Areas of cloud trying to advance from the west will add to the uncertainty in extent of fog later in the night and through Monday morning.”
Travellers posted pictures online of the fog...
The lowest temperature in the UK was in Katesbridge, Northern Ireland, where minus 6.9C was recorded.
The lowest temperature in England was in Northolt, near Heathrow, where a it was minus 6.2C (21F).
Visibility at Heathrow was as short as 100 yards this morning.
The west London hub is particularly badly affected by fog because it operates at 98% capacity, with a plane landing or taking off every 45 seconds.
This means it has less capacity to deal with delays than other airports.
According to BBC weather, temperatures dipped as low as -5C in the south of England.
A spokesperson for Heathrow said: “Foggy weather across the South East has reduced visibility at Heathrow.
“As a result, some passengers may experience disruption to their journeys today.”