Temperatures as low as -10C (14F) will increase the risk of breathing problems, heart attacks and stroke.
Elderly and vulnerable people are being advised to take extra care, stock up on medicines and make sure they have had a flu jab.
Sandy Brown, deputy chief executive of East of England Ambulance Service, said: “We expect to see an increase in calls and we have additional resources prepared.
“Everyone can play their part and we are calling on the public to check on vulnerable members of the community and make sure they have sufficient over-the-counter and prescribed medicines.”
An NHS spokesman said: “Evidence shows that the risk of heart attacks, strokes and serious breathing problems increases as temperatures plummet.”
Evidence shows that the risk of heart attacks, strokes and serious breathing problems increases as temperatures plummet
The cold snap will last at least until the middle of next week, with some forecasters warning of a longer icy spell.
The Met Office and Public Health England last night upgraded their existing level 2 health alert to level 3.
This is just one step below a national emergency and warns of a 90 per cent chance of “severe cold weather” across northern England until Tuesday.
It states: “Colder conditions will spread south across much of England during Saturday as a northeasterly airflow develops.
“This weather could increase the health risks to vulnerable patients and disrupt the delivery of services.
Temperatures as low as -10C could be deadly for vulnerable patients
“Prolonged periods of cold weather can be dangerous, especially for the very young, very old or those with chronic diseases.”
A lower-grade alert is in place across the rest of the country into the start of next week.
Freezing winds and rain across Scotland, northern England and along the east coast will turn to snow today with flurries possible at low levels.
Harsh frosts are expected and Met Office forecaster Emma Sharples said temperatures could drop to -10C in parts with widespread lows of -3C to -7C.
She said: “Everywhere is going to feel cold over the next few days as we go into a northerly airflow.
Elderly people are being advised to make sure they have had a flu jab
“We expect showers across Scotland and the North and along the east coast into the weekend and these will turn wintry.
“We could see some snow to low levels.
“It is also going to be windy with the risk of gales in the North and we expect temperatures of below -10C in exposed spots.
“Daytime temperatures are likely to remain in single figures although in parts they could stay at freezing or even just below.”
As the cold strikes, the nation’s hospitals are already swamped with people suffering “Aussie flu”.
Met Office forecaster Emma Sharples said temperatures could drop to -10C
Official figures reveal one in four admissions are now due to the lethal H3N2 bug, with patient numbers soaring from 1,281 to 3,751 in a week.
An 81-year-old woman died on Tuesday after waiting almost four hours for an ambulance, the GMB union said yesterday.
Paramedics eventually forced their way into her home in Clacton, Essex, but she was already dead.
The union’s regional officer Dave Powell said: “My concern is that we are suffering deaths while people wait for ambulances.
“It is another example of how we are not coping, I’m sure these cases are much more widespread than the public is aware of.”
Britain should enjoy a brief respite from the cold mid-week with signs of a brief return to milder, unsettled conditions.
As the cold strikes, the nation’s hospitals are already swamped with people suffering ‘Aussie flu’
However, some forecasters warn the next icy blast is not far behind, with wintry weather possible for the rest of the month.
James Madden, of Exacta Weather, said strong winds with heavy snow in rural spots could whip up blizzard conditions.
“This weekend will see a significant switch back to much colder temperatures and this will be accompanied by widespread wintry weather and snow showers,” he said.
“Northern regions can expect some heavy and settling snow across higher ground and lower levels.
“There is also the potential for some blizzard conditions and drifting snow in places.”
The threat of a severe cold snap has prompted warnings for people with health problems to stay indoors.
Dr Thomas Waite, of Public Health England, said: “As temperatures drop it’s really important to think about what you can do to prepare for the cold and protect more vulnerable friends and family.
“Ensure they wear lots of thin layers, have plenty of warm food and drinks in order to stay warm, and check weather forecasts before heading out.”
Dr Penny Woods, chief executive of the British Lung Foundation, said extreme winter weather can worsen asthma and heart conditions.
“For people living with lung conditions, cold weather can trigger a flare-up of symptoms,” she said.
The Met Office and Public Health England last night upgraded their existing health alert to level 3
“In particularly cold conditions, our advice is to check the weather before going out.
If you’re not feeling well or are having trouble breathing, stay indoors and keep warm.”
Malcolm Booth, chief executive of the National Federation of Occupational Pensioners, said: “People often get caught unawares by a sudden cold blast.
“It might be wise to stock up on basics so there is something warm to eat and will save you having to venture out if it turns very cold and snowy.”