LONDON (AFP) - A thick fog in London forced the cancellation of around 100 flights at Heathrow Airport on Monday (Jan 23) and revived concern about air quality in the capital.

"Foggy weather across the South East has reduced visibility at Heathrow," the airport said in a statement, adding that the cancellations amounted to around 8 per cent of scheduled flights.

There were also dozens of cancellations at London City Airport and road users were warned about driving conditions after freezing temperatures overnight.

A map from Britain's environment ministry showed "very high" pollution across southeastern England on Monday (Jan 23).

"Reduce physical exertion, particularly outdoors, especially if you experience symptoms such as cough or sore throat," the ministry's advice read.