A weather warning has been issued urging drivers to be wary of fog and sub-zero temperatures across swathes of England.
The UK Met Office said "freezing fog" had settled in patches of the country and poor visibility could persist into Friday afternoon.
Flights were held up at Heathrow and London City airports on Thursday because of poor visibility while Luton faced disruption after a technical issue in relation to air traffic control.
This morning, Met meteorologist Emma Sillitoe said: "The main thing we have had is large, dense freezing fog patches. "People driving around in the morning should take care on the roads."
Visibility could be as low as 100 metres and ice could develop on the roads, she added.
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The "yellow" fog warning, issued until 10am on Friday, affects much of southern England but patches of freezing fog had been witnessed from the Midlands and the Humber to the South West.
The fog is set to lift and thin slowly and could linger into the late morning and early afternoon, particularly in the South East and East Anglia, the Met Office said.
On Thursday, temperatures dipped to freezing levels in areas such as Suffolk, Gloucestershire, Cumbria, Devon and northern Scotland.
It follows a fatal pile-up on the A40 in treacherous conditions on Wednesday morning which left one woman dead and many injured.
The AA urged "extreme caution" for those heading out on the roads, adding that extra time should be allowed, tyres properly inflated and screens adequately de-iced.
Spokesman Ian Crowder said on Thursday: "We have got the worst possible conditions really, of fog and icy roads, and that can be lethal.
"I think the message is extreme caution and to prepare for the worst and make sure all windows are clear."