A health warning has been issued for the elderly and ailing as temperatures plummet and freezing fog disrupts travel across the country.
Public Health England (PHE) has issued its first “level three” cold weather alert since January.
Road travellers face the dual problem of fog and frost – a day after a motorist was killed in a major crash following foggy conditions on the A40.
Met Office forecaster Mark Wilson said visibility could be limited to 100m or less in some parts on Thursday morning.
The Met Office has issued yellow “be aware” fog warnings across the bulk of England and parts of Wales.
Forecasters see the unsettled, milder weather over Christmas making way for clearer but colder conditions as the new year approaches.
The cold weather warning was issued after plunging temperatures were forecast for northwest England, Yorkshire and the Humber, the Midlands and the east of England.
A level three alert is issued when average temperatures look set to fall to 2C and if either widespread ice or heavy snow is present. A level two warning remains in place in the rest of England.
Dr Thomas Waite, consultant in extreme events and health protection at Public Health England, 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.
“Cold does kill, even in places where the temperatures aren’t at their lowest.
“Most of the practical advice on keeping warm in cold weather may seem like common sense, but it’s important that we all help each other to stay well this winter.”
Dozens of flights were delayed or cancelled due to poor visibility at London City Airport on Wednesday morning, while a number of arrivals were diverted elsewhere.