Drivers are being warned of hazardous conditions on roads as heavy snowfall hits parts of the UK.
Three people died following a crash in Lincolnshire while one man died in a crash in Cambridgeshire. Police have not said if they were weather-related.
Hundreds of trains are cancelled, many schools are shut and forecasters say some rural communities may be cut off.
Kent, Surrey, Suffolk and Sussex have seen up to 10cm (4in) of snow, while north-east England has had up to 8cm.
Amber Met Office weather warnings have been in place in those areas – meaning a possible risk to life and property.
A fresh amber warning is in place for potentially “extreme” weather in Scotland on Tuesday.
A less severe yellow warning of snow remains in place for a much wider area, covering much of Scotland, England and Wales.
Lincolnshire Police say three people died following a crash at about 06:15 GMT on the A15 near Baston.
Officers have not said whether the crash, which involved a blue Renault Clio and a white lorry, was caused by the poor weather, but the force said earlier that roads in the county were “particularly hazardous”.
Lincolnshire County Council said the road had been gritted in the early hours of Tuesday.
A man also died following a three-car collision on the A47 near Peterborough, while 17 vehicles were involved in a crash on the A120, in Essex.
Officers in Lincolnshire also helped 45 schoolchildren from their bus after it collided with a car and left the road. None of the pupils was injured.
The UK is experiencing what is expected to be the coldest week of the winter, with snow moving west during Tuesday, spreading into Scotland and the Midlands.
Up to 40cm (15in) of snow could fall in north-east England and Scotland, between Wednesday morning and Thursday lunchtime, the Met Office says.
Scotland’s transport minister Humza Yousaf warned an amber warning in place for parts of the country could be upgraded to the most serious red warning – meaning “extreme weather is expected”.
Temperatures dropped well below freezing on Monday night, with Northern Ireland, south-west England and East Anglia logging figures as low as -6C. The lowest temperature was -8.9C and was recorded in Farnborough, Hampshire.
It is expected to get as low as -15C by midweek in areas where snow has fallen.
BBC Weather meteorologist Gemma Plumb said: “On Wednesday and Thursday it is going to feel bitterly cold with a lot of areas seeing maximum temperatures below freezing.
“There will be heavy snow showers in the East, particularly in north-east Britain.”
Among the disruption on Tuesday:
- Rail journeys have been badly affected, with Southeastern cancelling more than 100 services between Kent and London. Southern said it was running a reduced service, and the Gatwick Express is also disrupted
- In East Anglia, Great Northern and Greater Anglia services are also affected, with a reduced service being run on many lines. In Essex, the Stansted Express, Tfl rail and c2c services are running a reduced timetable
- Ice and snow on the roads has made driving conditions “treacherous” in some areas, according to police, with cars and lorries becoming stranded
- The M20 in Kent is gridlocked due to the weather, while Lincolnshire Police said officers had been called out to 20 collisions since 05:45 GMT – many more than on a typical day
- British Airways cancelled 60 short-haul flights to and from Heathrow in preparation for the poor weather
- Most other airports remain unaffected, but Liverpool John Lennon Airport’s single runway was briefly shut for a time due to snow
- Chester Zoo is closed due to the cold – a spokesperson said the animals’ welfare was “one of our top priorities”
Network Rail said it was working to restore rail services in East Anglia, after conditions had not been as bad as expected.
Some lines had been closed in advance of forecasted snow in the area, with engineers asked to focus on keeping the main lines open.
Meliha Duymaz, from Network Rail Anglia, apologised to passengers, saying the weather had not been as bad as anticipated and a full service was expected on Wednesday.
Motorists, meanwhile, are being urged to take extra care and in the worst affected areas and only travel if really necessary.
The RAC’s Pete Williams advised drivers to check fuel levels, oil, tyre tread and pressure, coolant, lights and screen wash before setting out.
Hundreds of schools across the UK are shut, particularly in north-east England, Yorkshire, and the South East.
More than 330 schools were closed in Kent, while closures have also been announced in north-west England and northern and eastern Scotland.
In north Wales, more than 230 schools were shut.
Weather warnings are expected to be in place until Friday.
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