The warning upgrade comes after temperatures dropped last night to this winter's record low, with forecasters recording -14.4C in Braemar, Aberdeenshire, not far from the historic low of -15.6C in Holbeach, Lincolnshire, in 2012.
The Met Office has extended its heavy snow and ice warnings from Wednesday night through to Friday. England and Wales should not be quite as cold.
The disruption caused by the snow is set to deepen with a weather system arriving from the South West and pushing north on Thursday.
According to the forecaster there is a 90 per cent chance of "heavy snowfall" from 8pm to 10pm.
Friday will be overcast and breezy with outbreaks of rain, sleet and snow.
The Met Office said rain and snow were expected to move into the United Kingdom from the southwest from Thursday and through Friday with icy patches forming, but that snow amounts would "vary markedly" across England and Wales. "Please stay with your vehicles and we will get to you". "It's looking like it will be a spell of persistent snow".More news: Apple revokes Google’s enterprise iOS certificate, shuts down internal apps
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Further rail and air travel delays are possible this evening and Friday, the Met Office warned, as it said London airports could face disruption.
Benton in Oxfordshire and Santon Downham, Suffolk, are among parts of England which could be coldest, and temperatures across southern areas are forecast to dip to around minus 8C (17.6F).
Coastal areas may also see the odd wintry shower, with a maximum temperature 2C.
It is set to fragment, but will leave an ice risk for many parts, they warn, making driving treacherous in the morning.
Motorists have been told to expect delays due to ice on the roads and some vehicles may become stranded in the snowy conditions.