The Met Office has issued a yellow weather warning for fog which could freeze in some places and disrupt transport, covering the eastern edge of Wales and most of England.
Temperatures are also set to dip to near zero overnight for many areas, meaning the fog could freeze on some roads.
The Met Office has said temperatures are around average for the time of year, with overnight temperatures of 0C (32F) in Cardiff.
Met Office forecaster Craig Snell said motorists should allow extra time for their journeys.
He said: “Not everywhere within that warning will be foggy, but the main message to drivers is to allow some extra time for your journey because there could be fog around.
“It could have a knock-on effect on public transport, so there could be delays to bus journeys and train journeys.
“The fog will tend to clear as the morning goes on.
“It may linger on in a few spots, but towards lunchtime conditions should be much better.”
Mr Snell added that a combination of long winter nights, clear skies and still air can cause fog to settle.
“When the air is quite still, there’s nothing to move the fog around so that’s why it becomes dense,” he said.
“This time of year, our nights are long so we don’t have the heat from the sun, and the ground starts to radiate heat from the surface.
“This creates a long period of cooling.
“It’s a mixture of clear skies, light winds and long nights.”