Public Health England has issued its first nationwide severe warning for cold weather since the Beast from the East hit the UK three years ago, as the elderly were warned to stay indoors.
Temperatures are set to drop below usual winter lows and blizzards are forecast in the south east, part of the most severe widespread cold conditions the country has seen since a "sudden stratospheric warming event" brought Siberian winds to the UK in February and March 2018.
England is entering the "coldest period of the winter so far", and people should prepare for up to five days of icy weather, with widespread extremely cold temperatures beginning Saturday morning and continuing until Wednesday evening, the Met Office said.
Below-freezing temperatures as low as -4C will feel even colder because of wind chill, forecasters said.
The rare national alert comes as the east part of the country braced for an influx of snow on Sunday which could disrupt travel, block roads and interrupt power supplies, particularly in the south east.
"Avoid exposing yourself to cold or icy outdoor conditions if you are at a higher risk of cold-related illness or falls," the "severe" Level 3 alert says.
People should also keep an eye on the weather forecast, make sure their heating is keeping their home at a temperature of at least 18C, wear appropriate shoes and clothes if going outdoors, and check on their elderly and vulnerable neighbours.
The warnings to stay at home and avoid going out unnecessarily echo calls already in place because of the Covid-19 pandemic.
Two such nationwide cold weather alerts were issued in March 2018, one as a direct result of the Beast from the East, and one because of continued cold weather later in March.
Tom Morgan, a meteorologist at the Met Office, said: "It's quite unusual, the set up that we're going to have next week, with so many areas seeing widespread low temperatures, and also quite a lot of snow. It's three years since we've had something similar."
Snowy conditions spread across Scotland on Thursday, with drivers rescued from their cars on Friday morning after becoming stranded in two-metre snowdrifts.
In Scotland blizzards are expected over high ground on Saturday, with the entire eastern part of the UK set to see some snow by Sunday evening.
Further south, up to 20cm of snow could fall in Essex, Kent, Suffolk and Norfolk over the weekend.
Jo McLellan, a spokeswoman for the Met Office, said: "That's an area of widespread, persistent, and occasionally quite heavy snow - mostly five to 10 centimeters, potential for 15 to 20 centimeters, but it's quite dusty.
"We might see some drifting snow and blizzards because of the strong winds there."
Dr Owen Landeg, group leader, extreme events and health protection at PHE, said: "Cold weather isn’t just uncomfortable, it can have a serious impact on health. For older people and those with heart and lung problems, it can increase the risks of heart attacks, strokes and chest infections."
The cold snap, dubbed the "Beast from the Baltic" because it comes from cold air moving in from Russia and Eastern Europe, is set to continue until Thursday next week, when milder temperatures are expected.
On Friday afternoon the Environment Agency had 35 flood warnings in place in England, including in the north east and on the Thames west of London, though water levels were beginning to drop in many places.
The Scottish Environmental Protection Agency had two flood warnings in place for areas near Perth because of snowmelt and persistent rainfall.
Experts warned that fast melting next week on already-saturated ground could cause the worst snowmelt flooding in the south east since the Thames burst its banks in 1951.
Professor Hannah Cloke, an expert in hydrology and flood forecasting at the University of Reading, said: "If the snow comes and rapidly melts, it's still very very wet and that won't have had a chance to drain away, then we will be looking at more flooding."