Scattered snow showers and freezing temperatures could cause icy conditions on roads in parts of the UK, according to forecasters.
The Met Office upgraded its cold weather alert to Level 3, stating that there was a 100% probability of "severe" conditions across most of England this week.
The warning means there is expected to be an impact on people's health and on health services, with a surge in demand for care.
According to the Met Office temperatures will drop to as low as -6C (21.2F) on Wednesday and Thursday, when daytime maximums will be no more than 3C (37.4F).
Severe weather warnings for ice were also issued for tonight and tomorrow morning across eastern parts of England and Scotland, and Northern Ireland, south-west England and south Wales.
A high pressure system hanging over Scandinavia and western Russia is pushing raw, easterly winds towards the UK, meaning this will be the longest spell of cold weather so far this winter, experts say.
Clare Allen, forecaster with MeteoGroup, the Press Association's weather division, said: "We've got an easterly wind across the country bringing snow showers off the North Sea.
"Most areas of the UK will see daytime maximum temperatures of 2C (35.6F) and 3C tomorrow.
"London will be a maximum of 4C (39.2F), so it's going to be pretty cold."
Ms Allen said the mercury could plummet to as low as -6C in rural parts of south Wales tomorrow night, and -5C (23F) in Devon.
"The minimum temperatures will be -3C (26.6F) and -2C (28.4F) across the country generally.
"If people aren't careful driving then it could cause disruption. Anywhere that is wet, it will definitely cause ice," she added.
A Department of Health spokesman said: "During this period of cold weather we would advise people to check local weather forecasts for the latest information on weather conditions and staying safe when outdoors.
"We are working closely with the NHS and local authorities to ensure vulnerable people stay well during cold weather and get the help they need within their communities."