The Met office issued a weather forecast for such dense fog this morning that driving conditions would be made more difficult and there was potential for travel disruptions.
The fog could be so thick that in some places the view was predicted to drop as low as 50 meters.
According to the Met Office, most of the fog was supposed to clear up around mid-morning, but forecasters warned that some stubborn stains could continue until later.
The yellow weather forecast affected the South West, East Midlands, London, South East and East of England and was to be lifted around noon.
The Met office warned of slower travel times, with delays to bus and train services, as well as the possibility of delays or cancellations of flights.
Motorists were advised to be extra careful and allow more time to travel.
In the fog advice posted on its website, the National Weather Service warned: “Things to keep in mind when planning your trip are that fog can be very localized, so it may not be clear when you take and it can affect air travel and roads in different ways. “
Elsewhere, a rainstorm that was once heavy across Scotland and Northern Ireland will slowly be edged to the south-east and become lighter and more uneven as it reaches northern England and Wales.
In other parts of the country, morning mists will be ready to leave a nice, warm afternoon.
Tonight could see low cloud with slightly uneven light rain, with potential for more fog.