The whole UK is set to be hit with a wave of snow and ice on Monday, the Met Office says.
It has issued yellow warnings for snow and ice for England, Northern Ireland, Scotland and Wales for Monday and Tuesday, with further warnings throughout the week until Friday.
The Met Office warned that temperatures could drop below minus 15C.
It brought forward its previous yellow warnings as the UK braces with the cold weather caused by a blast of Arctic air.
The Met Office said temperatures could drop below minus 10C on Monday night on higher ground in northern Scotland, with northerly winds making it feel even colder.
The Met Office said temperatures could drop as low as -15 overnight on Tuesday in some sheltered Scottish Glens.
In an update on Monday, the Met Office said temperatures in parts of the UK will drop below freezing on Monday night and will not top mid-single figures during the day on Tuesday or Wednesday.
Up to 30cm of snow could fall in isolated spots across Scotland while other areas could see 10-20cm of snow fall.
A yellow weather warning will be in place for London, the Thames Valley, East Anglia, parts of the Midlands and South Wales from 9pm on Monday night until 10am on Tuesday.
An identical one will be in place in Northern Ireland at the same time.
A yellow weather warning for snow and ice is already in place in much of Northern and Eastern Scotland and North East England.
Watch: Cold weather alert as freezing temperatures hit the UK
The area covered by it is set to extend as far south as Hull from Tuesday before it lifts at 10am on Wednesday.
Overnight temperatures later in the week could reach minus 4C in London and minus 6C in Birmingham and Belfast.
Met Office chief meteorologist Dan Suri said: “Snow, ice and low temperatures are the main themes of this week’s forecast, as the UK comes under the influence of an arctic maritime airmass as cold air moves in from the north.
“Snow is already falling in parts of the north where some travel disruption likely, as well as a chance of some rural communities being cut off. Snow showers will continue through today and Tuesday here, and Northern Ireland will also be subject to some snow showers, especially over high ground.
The UK Health Security Agency (UKHSA) has issued a cold weather alert and has placed north-east England, north-west England and Yorkshire under a level three alert, with the rest of England at level two.
A level three alert means there is a 90% chance of severely cold weather, icy conditions or heavy snow, which could increase the health risk to vulnerable patients, the NHS said.
Areas affected by the warnings could experience power outages, delays to road, rail and air travel, icy surfaces and some rural communities may be cut off by the freezing conditions.
Dr Agostinho Sousa, head of extreme events and health protection at UKHSA, said: “During periods like this, it is important to check in on family, friends and relatives who may be more vulnerable to the cold weather, as it can have a serious impact on health.
“If you have a pre-existing medical condition or are over the age of 65, it is important to try and heat your home to at least 18C if you can.”
Last month, the Met Office suggested that in some areas of the country, there was "a small possibility of more organised rain or snow spreading southwards" in early March.
This is down to a phenomenon known as "sudden stratospheric warming".
Here, Yahoo News explains how it works.
What is sudden stratospheric warming?
Sudden stratospheric warming (SSW), refers to a rapid warming – up to about 50C in just a couple of days – high up in the Earth's stratosphere.
This is so high up that we don’t feel the ‘warming’ ourselves, but it can cause a knock-on effect a few weeks later, affecting the weather we experience lower down in the troposphere.
Every year in winter, strong westerly winds circle around the Arctic high up in the stratosphere – something known as a stratospheric polar vortex, the Met Office explains.
Sometimes – although not every year – this causes winds in the polar vortex to weaken, or even reverse to flow from east to west.
Cold air then descends rapidly in the polar vortex, causing temperatures in the stratosphere to soar by as much as 50C in the space of a few days.
Cooler air from the higher level disperses and sinks down to the troposphere, which can cause the jet stream to change shape and "snake" across the North Atlantic and northern Europe, including Britain.
A sudden stratospheric warming event was linked to the Beast from the East that swept the UK in 2018.
What is the Beast from the East?
The Beast from the East, a wintry storm that ran from around 22 February to 5 March 2018, saw weeks of heavy snow and wind pelting the UK and elsewhere in Europe.
It resulted in a total of 17 deaths in the UK, with 22 inches of snowfall in some areas and £1.2bn in damage.
Since then, the term has been used in the media to describe subsequent spells of heavy snow.
Although the blizzard was caused by a SSW event, the Met Office says there is a “low probability of having widely disruptive winter weather” like that of March 2018.
Is there a sudden stratospheric warming taking place now?
A major SSW took place some 50km above the North Pole earlier this month, causing the stratospheric polar vortex to reverse.
This weather event is currently still taking place, although as mentioned, there is usually a delay before people start to feel it on the Earth's surface.
The cold snap is expected to arrive within the next two weeks, according to the Met Office, with “much colder” weather expected in the middle of March.
Its long-range forecast covering 9-23 March says: "Spells of rain or snow, are more likely than earlier in the month, with a low chance that some wintry episodes could be disruptive, though northwestern areas most likely to see the driest conditions.
"Winds could often be from a northerly or easterly direction, and temperatures are more likely to be below-average than above-average overall, but later in the month, colder air will be fighting against a strengthening sun."