Dense patches of fog with visibility dropping below 100 metres at times are behind a yellow warning covering parts of England and Wales on Saturday morning.
The Met Office has told people to be ready for conditions to change, even though the warning runs only until 10am on Saturday.
In some spots, the fog may not clear until the afternoon, the weather office warned.
Drivers were told to expect traffic delays because of the fog.
Anyone flying was advised to plan for delayed or cancelled flights.
The Met Office said: “Areas of fog, dense in places with visibility below 100 metres at times, will slowly begin to thin, lift and clear later this morning or, in some spots, not until this afternoon.
“Bus and train services, as well as flights and ferry travel may also be affected; check for updates from your travel company and follow their advice.”
Before setting out, the weather bureau recommended checking road conditions.
It also advised making sure fog lights were working.
The warning area includes parts of the East and West Midlands, the East of England and Yorkshire and Humber.
Buckinghamshire, Milton Keynes and Cheshire are also covered.
In Wales, the warning takes in areas around Denbighshire, Flintshire, Powys and Wrexham.
The fog follows days of heavy rain across England, which pushed river levels up in many parts of the country.
Storms Austen and Boelo gave England its wettest week since October 2024.
Austen brought an amber Met Office warning for strong winds and rain across Cornwall and parts of Devon on Friday October 23.
A yellow warning covered much of central and southern England at the same time.
Gusts of nearly 80mph were recorded on the Isle of Wight.
Flights were cancelled and thousands of homes lost power.
Boelo arrived the next day, Saturday October 24, prompting an amber warning for wind over Scotland and a yellow warning covering northern England.
Despite the recent rain, much of the country remains officially in drought.
A severe drought is in place across parts of the South West and Sussex.
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