Temperatures plunged to below minus 22C overnight, the lowest in the UK in more than two decades, following an “extreme freeze”.
The mercury dropped to minus 22.9C (minus 9.2F) in Braemar, Aberdeenshire, the coldest temperature recorded in the UK since 1995, the Met Office said.
Forecasters said the last time a temperature below minus 20C was recorded in the UK was December 23 2010.
Scattered snow showers and some sunshine is forecast for large swathes of the UK over the rest of the day on Thursday.
Yellow weather warnings for areas where wintry weather could cause travel disruption have been issued for the morning, covering the eastern half of Scotland and England, Devon, and south-west Wales.
As the snow clears towards the east through the morning, it could become “quite pleasant outside with the sunshine, although still bitterly cold” in western areas, according to Met Office meteorologist Clare Nasir.
The latest plunge in temperatures comes after Tuesday night, when minus 17.1C (1.2F) was recorded in the Scottish Highlands.
That marked the coldest temperature recorded in the UK since January 2010, when the mercury dropped to minus 22.3C (minus 8.1F) at Altnaharra in the Highlands.
Cities such as Manchester and Carlisle may drop to minus 4C (24.8F) on Thursday, and York is expected to have lows of minus 6C (21.2F).
Nine flood warnings, where flooding is likely, and 88 flood alerts, indicating that flooding is possible, have also been issued for locations across England by the Environment Agency, but none were in place for the other UK nations on Wednesday night.
The forecast prompted responses from local councils in England, with residents warned to take “extra care” while the cold snap lasts.
Ms Nasir said that on Friday and over the weekend the weather is expected to turn “quite hazardous” as “an area of cloud and rain” coupled with extremely cold air brings further risk of snow, and “perhaps even of freezing rain”.
The Centrepoint charity for homeless youth has urged people to contact them or Streetlink on 0300 500 0914 if they see people living out in the cold.
Charities have also advised offering anyone sleeping rough a hot drink, meal, blankets or clothing.
The NHS said that, despite the tough conditions, many vaccination centres have remained open, and any missed appointments will be rescheduled.
Several councils in England, including Rugby Borough Council, and Southwark Council in London, were forced to briefly suspend waste collection services due to the weather conditions.
Thurrock Council in Essex said more than 100 workers, mostly from street cleaning and waste collection teams, were diverted from regular duties to help clear snow from footpaths and routes to medical centres.
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