Temperatures will plunge to -9C across parts of southern England overnight, the Met Office said on Monday, with the freezing weather expected to last until the end of next week.
Freezing fog will remain in some areas on Tuesday morning, as the cold snap shows no sign of abating.
A level 3 cold weather alert has been extended until 9am on Friday by the UK Health Security Agency (UKHSA), with people urged to keep warm and check on vulnerable family and friends.
The Met Office forecaster Craig Snell told the PA news agency: “We’ve got rather cloudy, damp but mild weather across Northern Ireland, Scotland and far north-west areas of England on Tuesday. Temperatures could get up to 12C or 13C across Aberdeenshire way.
“Farther south across the rest of England and Wales, we’re still under the colder conditions, so there could be some fog again which will be quite slow to clear, so a very similar start to Monday.
“When there is cloud, it’s going to feel pretty cold. But in the sunshine, although it will be cold, the sun will help negate the cold a little bit, so all in all a pretty similar day to Monday.”
With temperatures plummeting to -9.5C in Santon Downham, Suffolk, on Sunday, it was confirmed hundreds of households in England and Wales are eligible for cold weather payments. These payments are made to vulnerable people to help them with heating bills when the temperature dips below freezing.
Payments go to those living in an area where the average temperature is recorded as, or forecast to be, 0C or below over seven consecutive days.
Payments will be made to homes across north-east England, Cumbria, west Wales and Oxfordshire.
Snell added: “Tuesday morning will probably be the coldest, with some parts of southern England getting down to -9C, hovering around -7C in the early hours of Wednesday.”
Meanwhile, the Environment Agency has 34 flood warnings in place, meaning flooding is expected and action to prevent it should take place.
Sadiq Khan, the mayor of London, also issued a high air pollution alert for the capital for Tuesday.