Temperatures plunged to almost -7C overnight as a cold snap deepened - and health experts have issued an urgent warning about freezing weather.
The coldest place in the UK was Topcliffe in North Yorkshire, where it was a frigid -6.8C in the early hours of Sunday.
It was followed by Benson, Oxfordshire (-6.2C), Katesbridge in Co Down, Northern Ireland (-5.9C) and Derrylin in Co Fermanagh (-5.5C).
The Met Office said Northern Ireland experienced its coldest night of the winter, while temperatures of -1C were felt across London - and the chilly conditions are set to continue this week amid a warning from Public Health England.
Met Office meteorologist Alex Burkhill said a fog warning had been put in place over sections of the West Midlands and north-west England.
In many places, Britons woke up to their cars and gardens covered in frost.
Mr Burkhill said: "It's been a very frosty start but through the bulk of the day it's going to be largely fine and largely sunny."
It would still be "quite chilly" with expected highs of 8C scattered across parts of England, Mr Burkhill added.
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He said that there would be some rain for the Shetland Islands.
Rain is also possible near coasts in south-eastern places.
Temperatures will peak at about 6-7C across the country.
Meanwhile, the fog warning is in place for parts of England, including Manchester, Liverpool and Birmingham, and North Wales until 1pm on Sunday.
A mid-range cold weather alert has been issued by Public Health England from 6pm on Sunday to 6pm on Tuesday.
The organisation is urging people to prepare for cold weather conditions and look out for those most at risk, especially the elderly.
Dr Owen Landeg, principal environmental public health scientist at PHE, said: "Below 18 degrees, changes to the body mean that the risk of strokes, heart attacks and chest infections increase so heating homes to this temperature is particularly important to stay well."
Police have urged drivers not to leave their vehicles to defrost after an unattended car was stolen in Hertfordshire.
The vehicle was taken after it was left with its engine running on a driveway in Hemel Hempstead on Saturday morning.
It was later found on its roof after crashing nearby
Met Office five-day weather forecast
Sunday
Fog patches in the west could persist all day in a few places.
Otherwise, mostly dry with sunny periods after a frosty start.
The odd shower near coasts in the south-east, whilst cloudier and windier in the far north.
Tonight
Frost and patchy fog in southern areas, whilst cloudier skies further north bringing some rain or drizzle to the far north and north-west, where it will also be windy.
Monday
Dry with sunny spells in the south after frost and fog lifts.
Cloudier in north-western parts with some rain in the far north, where it will be windy, but milder.
Tuesday to Thursday
Low cloud and patchy rain and drizzle will often affect central and north-western areas; it will be bright at times in the north-east, while frost and fog plagues the south.