The Met Office has issued a weather warning for fog, with difficult driving conditions and travel disruption expected.
Fog patches may become so dense, visibility could dip as low as 50 metres in some places.
Most of the fog will clear around mid-morning on Saturday, but one or two stubborn patches may persist until later on.
The yellow weather warning affects the South West, East Midlands, London, the South East and East of England, and will remain in place until about 10am on Saturday.
The Met Office has warned of slower journey times, with delays to bus and train services possible, as well as a chance of delays or cancellations to flights.
Motorists are advised to be extra careful and allow more time for travelling.
In advice about fog published on its website, the national weather service warns: “Things to bear in mind when planning your journey are that fog can be very localised, so may not be apparent when you set off, and it can affect air travel and the roads in different ways.”
Elsewhere, a band of rain, heavy at first across Scotland and Northern Ireland, will edge slowly towards the South East, getting lighter and patchier as it reaches northern England and Wales.
In other parts of the country, morning fog patches will clear to leave a fine, warm afternoon.
Tonight could see low cloud with some patchy light rain, with the potential for more patches of fog.