The Met Office with Public Health England (PHE) upgraded a level-2 cold weather alert to a more serious level-3 ‘cold weather action’.
The warning, which remains in place until Thursday, urges the elderly and vulnerable to take extra care during severe weather.
Temperatures could drop as low as -10C (14F) in parts of the country over the coming nights as much of Britain dips below freezing.
It comes as official figures reveal the number of people hospitalised due to flu has doubled since the end of last year.
Extreme cold could encourage the virus, which includes the deadly ‘Aussie Flu’ strain, to spread, experts warn.
Health officials RAMP UP warnings amid IMMINENT big freeze
Certain infections are more common during the winter, particularly influenza and norovirus
PHE’s acting head of respiratory diseases Dr Richard Pebody (correct) said cold weather can drive a surge in norovirus winter vomiting cases.
He said: “Certain infections are more common during the winter, particularly influenza and norovirus.
“This is due to a combination of factors, but includes factors to do with the bugs themselves, and the fact that they may spread more easily when we spend more time together indoors.
“Influenza is responsible for a number of deaths each year and affects the same vulnerable groups as cold weather.”
Dr Martin Wiselka, infectious diseases consultant at the University Hospitals of Leicester NHS Trust added: “We find that people huddle together during periods of cold weather and spend more time indoors.
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“Flu does seem to spread more in the cold weather and we think it is due to many people in close proximity to each other encouraging the flu to spread from person to person.
“It only takes one person carrying the virus to pass it on to a room full of people through droplets in the air.
“Cold weather can also put extra stress on the immune system in elderly people if their homes are not heated properly and this can make them more susceptible to flu and pneumonia.”
Public Health England (PHE) records reveal 93 people have died of flu since autumn with 27 deaths reported during the first week of January.
There were 758 admissions to hospital during the first week of January compared to 421 in the final week of December.
On Wednesday night rain will move across England, brining milder air
Professor Paul Cosford, Public Health England’s (PHE) medical director, said: “Our data shows that more people are visiting GPs with flu symptoms and we are seeing more people admitted to hospital with flu.
“We are currently seeing a mix of flu types, including the A(H3N2) strain that circulated last winter in the UK and then in Australia.
“We encourage anyone who is eligible to take up their offer of the flu vaccine – it is not too late.”
A Met Office and PHE health alert predicts a 90-per cent probability of severe cold, snow, ice and bitter gales until Thursday morning.
Heavy snow and strong winds could hinder public services while putting vulnerable people at increased risk, it warns.
Heavy snow and strong winds could hinder public services while putting vulnerable people at increase
The alert states: “Showers, heavy at times, will be most frequent in the north and west and more isolated in the south and east.
“These will fall increasingly as sleet and snow in northern England by Tuesday.
“Further south, snow will largely be confined to hills, perhaps reaching lower levels for a time on Tuesday evening and night.
“On Wednesday night heavy rain and gales or severe gales are likely to move across England, bringing milder air.
“There is a small possibility of snow across the north of England as rain meets the colder air, and this risk will be monitored.”
PHE figures reveal there were 37 GP consultations for flu per 100,000 patients in the first week of January compared to 21 the previous week.
Some 240 people were admitted to intensive care or high dependency units across the UK with influenza.