The highest level of weather warning has been issued for Scotland and Ireland as forecasters warn of “blizzard-like” conditions.
A red alert for Munster and Leinster in the Republic of Ireland and the central belt of Scotland has been put out for heavy snow and strong winds.
Dublin, Kildare, Louth, Wicklow and Meath are also covered by a red warning, which was issued early on Wednesday morning.
The Met Office definition of a red warning is that “you should take action now to keep yourself and others safe from the impact of the weather”.
Public Health England (PHE) has urged people to plan ahead to ensure they have enough food and medicine.
Police across the country advised motorists to avoid driving if possible owing to poor visibility and treacherous conditions.
Three people were killed in a crash in Lincolnshire and a man died after a collision in Cambridgeshire on Tuesday morning.
There were several road accidents on Tuesday night and in the early hours of Wednesday and a number of roads have been cleared and reopened.
Disruption is expected to continue across rail operators c2c, CrossCountry, Northern, Greater Anglia and Stansted Express, Merseyrail, ScotRail, Southeastern, Southern, Virgin, Thameslink and Gatwick Express, South Western Railway, TfL Rail and London Overground.
Commuters are urged to check their routes before travelling.
Farnborough in Hampshire recorded a low of minus 11C (12.2F) on Tuesday night, but for most places the mercury hovered at between minus 4C (24.8F) and minus 7C (19.4F).
Forecasters predict the snow and freezing temperatures will continue, prompting the Met Office to extend some of its weather warnings.
The red warning for parts of Scotland is in place from 3pm Wednesday to 10am Thursday.
Amber warnings covering parts of Wales, England and Scotland have been issued.
A yellow warning covering vast swathes of the UK has been extended until 9am on Saturday.
Glasgow and Newcastle Airports said teams were busy clearing the runways to keep schedules running on time, and advised passengers to check the latest flight information.
Heathrow and Southend Airports in London said it would largely be business as usual, though British Airways said the weather was likely to continue disrupting flights throughout the week.
Heathrow said flight schedules will be reduced on Wednesdayand that the rest of week is under review.
It is expected that the mercury could plummet to minus 15C (5F) by midweek where there is snow on the ground, rivalling temperatures forecast for parts of northern Norway and Iceland.
Rural communities could be cut off for several days, while power cuts and mobile phone network interruptions may occur, the Met Office said.
From Thursday, forecasters predict that another weather system, Storm Emma, will bring blizzards, gales and sleet as it meets the chilly “Beast from the East” later this week.