The Met Office has issued an amber severe weather warning for snow as all regions recorded their lowest temperatures for the winter so far.
The amber warning – which was issued for the South West – means the possibility of risk to life and property, travel delays, road and rail closures and interruption to power supplies.
The warning also means some rural communities could be cut off.
People are advised to be prepared to change plans and protect themselves.
“Three to seven centimetres is likely to accumulate quickly – within two to three hours – with up to 10cm (3.9in) in some places,” the Met Office said.
“The highest snowfall accumulations are likely to be in areas above 150 metres or so.”
It comes as the temperature in Braemar, Scotland, dipped to -14.4C (6.08F) this morning – the lowest UK temperature since 11 February 2012 when it fell to -15.6C (3.92F) at Holbeach in Lincolnshire.
Most of the UK is covered by a less severe yellow warning – which means be aware – as it prepares for more snow, ice, fog and strong winds.
England, Wales and Northern Ireland also had their coldest start of winter so far, with the temperature in Reedsdale Camp, northern England, dipping to -10.5C (13.1F).
Sennybridge in Wales was recorded at -9.3C (15.26F) while Katesbridge in Northern Ireland saw a low of -8.2C (17.24F).
Last year, the lowest temperature of winter was -14.2°C (6.44F) at Faversham on 28 February.
The Met Office has issued a yellow warning for snow, ice and fog, indicating minor disruption and risk, across almost all of the UK today.
Thursday could bring “very significant” snow, Met Office meteorologist Alex Burkill said, with up to 10cm (3.9in) expected to settle in areas of higher ground. Lower-lying areas could also get several centimetres.
Mr Burkill added: “It’s currently just a yellow warning, but it’s not out of the question that will be ramped up nearer the time.
“It’s looking like it will be a spell of persistent snow.”
Sky News weather presenter Nazaneen Ghaffar said: “The Met Office and Met Eireann have yellow warnings in force for most of England, Wales and Ireland for the risk of snow and ice on Thursday as a band of rain spreads slowly northwards to central and southern parts, hitting the cold air and turning to sleet and snow.
“Not everywhere will see snow, but where it does accumulate there is likely to be a couple of centimetres widely at low levels and possibly up to 10cm over high ground.
“Ice will also be a big problem on Thursday morning and by the evening snow will be an added hazard for central and southern parts of the UK and Ireland. Winds will also be strong across the South West.”
The wintry conditions are expected to last well into Friday with warnings of ice and snow likely to bring travel disruption over parts of north and east Scotland and England.
Councils have prepared for heavy snow, stocking up with more than 1.4 million tonnes of salt, the Local Government Association said.
The weather has already caused widespread disruption this week.
Thick fog has blanketed some parts of the country as drivers are being reminded to take extra care and use the correct lights on the M90 around the Queensferry Crossing in Scotland.
Ice on railways has caused widespread disruption, with Southeastern Trains announcing that 21 services were being cancelled or altered.
East Midlands Trains advised travellers to allow extra time to stay safe in the icy conditions amid delays as cold weather is forecast throughout Thursday.
Liverpool John Lennon Airport and Manchester Airport closed their runways for most of Wednesday morning as they struggled to shift snow and ice from their runways, delaying dozens of flights.
The deepest snowfall recorded by the Met Office at 9am on Wednesday was 11cm (4.3in) at Tulloch Bridge in Inverness-shire, which was also the coldest place, along with Leeming in North Yorkshire at -5C (23F).
Schools across Scotland, the North and Northern Ireland were closed due to the severe weather, and many are going into their second day of closures.
Roads in those areas, as well as parts of Devon and Cornwall, were also badly hit, with gritters and snow ploughs unable to work quickly enough to clear the snow and ice.
There was an increased number of accidents as motorists struggled to manoeuvre the snowy roads.
The temperatures in the UK, however, pale in comparison to the Antarctic-like temperatures forecast for parts of America.
The Midwest has seen a low of -46C (-50.8F) when wind chill is accounted for. Authorities are warning that in such conditions frostbite could take hold in minutes.