This week's cold snap could have a "serious impact on health", potentially leading to strokes and heart attacks, medics have warned.
The UK Health Security Agency (UKHSA) is urging anyone with a pre-existing condition, and the elderly, to heat their homes to at least 18C if possible.
Treacherous conditions are possible until Friday, with temperatures "well below average for the time of year", the Met Office has said, the mercury dropping below zero overnight.
It has yellow warnings in place for snow and ice covering northwest England and Northern Ireland until midday on Tuesday.
A similar warning covers large parts of northern Scotland until 9am on Wednesday, while there is a warning for ice in Devon and Cornwall for the first nine hours of Tuesday.
"Cold weather can have a serious impact on health, particularly older people and those with pre-existing health conditions, as it increases the risks of heart attacks, strokes and chest infections," said the UKHSA's head of extreme events and health protection, Dr Agostinho Sousa.
Jason Kelly, chief meteorologist at the Met Office, said ice is "likely to be a hazard for much of the week".
He added: "Cold, icy and sometimes snowy conditions are in the forecast with the UK seeing more of a northwesterly regime, with temperatures well below average for the time of year.
"Wintry showers are likely at points through the week in the north of England.
"Maximum temperatures will largely only reach the low single figures, with temperatures below freezing through most nights this week."
People are advised to heat a living room during the day and their bedroom just before going to sleep.
Met Office senior meteorologist Alex Burkill said the weather will become "more changeable" on Friday ahead of "milder" conditions.
"By the weekend there will be a real upturn in temperatures," he added.