The heavy fog is expected to largely lift ahead of New Year's Eve fireworks displays.
But the Met Office fog warning across the South of England has been extended until 10am on Saturday, with drivers being urged to take care.
Air passengers are also being warned of cancellations and delays in a further day of travel chaos.
Meanwhile, Public Health England (PHE) warned of an incoming cold snap with temperatures expected to plummet as low as minus 5C (23F) on New Year's Day.
A "yellow" weather warning was also issued in north-west Scotland, with heavy rain and gale-force winds expected to bring difficult driving conditions ahead of Hogmanay celebrations.
Met meteorologist Mark Wilson said visibility in the morning could be as low as 100 yards (metres) after the lingering, freezing fog spread further overnight.
He said: "We think it will gradually improve throughout the morning but it will be a slow process."
Some patches of fog could persist into the evening "but not so much, not as widespread or as dense".
Mr Wilson added that temperatures could drop to minus 5C (23F) in the evening on New Year's Day but the "short-lived cold spell" should end after the bank holiday.
He said: "It will turn colder though tonight and New Year's Day and Bank Holiday Monday but also there will be more sunshine to enjoy."
Dr Deborah Turbitt, from PHE London, told people to look out for others, particularly the very young, old and ill, during the cold snap.
She said: "Every winter thousands of people die because of their exposure to cold weather and doctors' surgeries, hospitals and other parts of the NHS are kept busy as people fall ill - that's why it's really important that we all do everything we can to ensure everyone stays well this winter."
A driver and 16 of his passengers were hurt when a coach veered off an M40 slip road and overturned in heavy fog near Milton Common in Oxfordshire at 2.45am on Friday.
It happened 20 miles (32km) from the scene of a fatal pile-up on the A40 in similarly treacherous conditions on Wednesday morning.
The blanket of fog covering the South East on Friday morning caused cancellations and delays at major airports, including Heathrow, where around 50 flights were cancelled.
Gatwick and London City Airport also experienced disruption because of the fog, and passengers were urged to check flight information in advance.
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