A cold weather alert is in place for large parts of England until 6pm on Tuesday as people are warned about particularly cold overnight temperatures.
Met Office forecasters say there is a 70% chance of severe conditions, bringing health risks to those most vulnerable.
The regions covered by the level two yellow alert are southwest England, southeast England, the East of England, East Midlands and the West Midlands.
Last night, temperatures dropped to -6.8C (19.7F) in Topcliffe in North Yorkshire and they are expected to fall again to -6C or -7C overnight into Monday.
Northern Ireland last night recorded its coldest night of the winter, which was -6.2C (20.8F) in Katesbridge.
Public Health England (PHE) has said older people and those with heart and lung problems are at most at risk of getting sick in cold weather.
PHE is urging people to keep an eye out for those who may need help staying warm, making sure they wear lots of thin layers and have everything they need.
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It says heating homes to at least 18C (64.4F) is particularly important as below this temperature, changes to the body mean the risk of strokes, heart attacks and chest infections increase.
More cloud coverage over other regions, including northern England, means temperatures there will not be as low.
The cold overnight temperatures will be offset by generally fine daytime conditions with sunny spells.
There has also been a warning of fog across parts of northwest England and the West Midlands until midnight.
Meanwhile, alongside freezing temperatures, forecasters have predicted air pressure as high as 1050 millibars into Monday.
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