A public health warning has been issued as Britain is set to plunged into sub-zero temperatures and freezing fog.
Elderly people and those with long-term illness have been urged to take precautions as some areas will see temperatures plummet to -6C.
A fog warning covering swathes of east Wales, the Midlands and eastern England is now in place, as areas across central and southern England have dealt with patches of fog and plunging temperatures.
Dr Thomas Waite, consultant in extreme events and health protection at PHE, said: “The effects of cold can be severe, in particular for those who are over 65, have a long-term illness, or are not mobile.
“Our advice to these groups is when indoors, have plenty of warm food and drinks and try to maintain indoor temperatures to at least 18C.
“If mobility isn’t an issue, keep active as best you can. If you need to go out wear lots of thin layers, and shoes with a good, slip-resistant grip to prevent any accidental falls.
“It is particularly cold at night this week, so drawing the curtains at dusk will keep the heat in.
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Fog blankets parts of Britain
“He also stressed the importance of checking on potentially vulnerable family and friends during the cold period.
He said: “We are expecting widespread frosts and fog overnight in central, southern and eastern regions which will be slow to clear and will lead to some quite chilly days.
Travellers should expect difficult driving conditions, longer trips and possible delays to their flights on Thursday.
A Met Office spokesman said: “Combined with temperatures below freezing, there is also the risk of ice forming on some untreated surfaces particularly later in the night and early on Thursday morning.
“The fog patches are expected to slowly thin during Thursday, although some patches may persist throughout the day – most likely in a zone from the West Country to eastern England.”
Dozens of flights were delayed or cancelled due to poor visibility at London City Airport on Wednesday morning, while a string of arrivals were diverted elsewhere.
A major crash on the A40, which killed one female driver and injured around a dozen others, happened after foggy conditions made it “really dangerous”, one witness said.
A motorist on the A40 who witnessed the fatal pile-up said that the poor visibility meant it was difficult for cars to see each other before they were “on top of it”.