Parts of the UK recorded the lowest temperatures of the year so far, with the mercury dropping to almost -10C in places.
The worst of the weather was in rural Scotland, with -9.9C recorded in Braemar, Aberdeenshire.
It exceeded the lowest temperatures of autumn so far, with -8.1C recorded in Tulloch Bridge on Sunday night.
In Northern Ireland, there were lows of -5.1C in County Down.
Fog affected the Thames Valley area with tricky driving conditions on the M4 West of London.
According to the Met Office, last night was the coldest night of the autumn so far.
"There is also plenty of freezing fog across parts of England and east Wales, stay #weatheraware."
Public Health England issued a cold weather warning which is in force until 5pm on Wednesday.
It is urging people to look out for anyone who may be in need of help to stay warm.
Dr Emer O'Connell, a consultant at Public Health England, advised people to look out for anyone who may need help staying warm.
"Older people and those with heart and lung problems are at risk of getting sick in cold weather," she said.
She said below 18 degrees, changes to the body mean the risk of strokes, heart attacks and chest infections increases so heating homes to this temperature is "particularly important to stay well."
Meanwhile, the Met Office has issued a yellow weather warning for fog until 11am on Tuesday, with a possibility of flight delays or cancellations and bus and train services facing disruption.
It said "some dense and freezing fog patches on Tuesday morning likely leading to some travel disruption."
The area affected includes Manchester, Liverpool, Wirral, Stoke, Nottingham, Birmingham and the Midlands and from Bath to London and on the South Coast and Kent.
The cold snap will end soon as temperatures are expected to rise throughout the week.
Large swathes of South Yorkshire and Derbyshire remain affected by flooding, as problems continue this week as the clear-up operation continues.
There are 34 flood warnings in place, but no severe flood warnings - which indicate a danger to life.
There could be more rain later today, coming in from the west.
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Met Office chief meteorologist Steve Ramsdale said: "After a very wet period for parts of the UK, the weather is now turning much quieter for a few days.
"With these quieter conditions, night-time temperatures will fall away with more widespread frosts than seen recently, despite daytime temperatures remaining fairly similar."
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