The fog alert came into force on Sunday evening and runs until 11am on Monday.
It stretches from York to Bath and covers most of the South East and eastern England, as well as parts of Wales and Northern Ireland.
The Met Office said travellers could face slower journey times or delays on public transport. There could also be disruption to flights.
Drivers could also have a tricky commute this morning and are being urged to allow extra time and make sure to use fog lights when visibility is under 100m
A bigger distance should also be left from the car in front, and the Met Office said full beam should be avoided because fog reflects the light back.
On Saturday, a stretch of the M20 in Kent was shut, and two people were seriously injured, after more than 30 vehicles were involved in a crash in thick fog.
There could be patchy fog even outside the warning area, “just not quite as dense or as impactful”, warned Met Office meteorologist Niamh Murray.
“Parts of the West Country could still see fog, Northern Ireland, really anywhere across much of the UK, even into the central belt of Scotland,” said Ms Murray.
She added: “We just have the perfect recipe at the moment: light winds, clear skies, long nights, to allow that fog to develop. And particularly if we see the negative numbers, it can quite stubborn to clear the next day.”
Fog may be particularly stubborn in rural parts of the Midlands, Wales and East Anglia, where temperatures were forecast to drop close to freezing overnight.
Visibility could be down to 50m or less in some places, the Met Office warned.
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Sky News forecaster Kirsty McCabe said dense fog was likely to cause issues over the next couple of nights – and that smoke from bonfires and fireworks could make visibility worse.
“Later next week we could well be talking about frost instead of fog, with the winds set to swing round to a northerly direction,” added McCabe.
“An area of high pressure sitting over the UK this week has effectively trapped all the mist and murk, making it harder for the fog to clear.”
