Forecasters have issued a danger to life ice warning as temperatures plummet to as low as -9C tonight.
If the mercury drops to -9C it will be the coldest night of the season so far, the Met Office said.
An ice alert for the Highlands, Orkney and Shetland islands is in force today, with the worst of the weather expected to end by 10am.
The Met Office warning states: "Scattered heavy showers will fall onto freezing surfaces overnight and Monday morning. These could wash off any surface treatment and lead to the formation of icy surfaces."
It adds that icy surfaces could lead to people injuring themselves by slipping over.
Met Office meteorologist Alex Burkill told the Daily Express : “Temperatures could dip to -8C (17.6F) or -9C (15.8F).... on Monday night.
“This would be the coldest night of autumn with the lowest temperature we have seen so far being -8.1C (17.42F).”
A netweather.tv spokesman added: "After a day with highs of 4-9c (coldest in the north), temperatures will quickly fall during the evening bringing a widespread frost and a few fog patches overnight."
The Environment Agency has 51 flood warnings and 90 less serious flood alerts in force for England.
Tonight will be dry for many with just a few scattered showers expected along the east coast.
Many will see clear skies too, with just the odd patch of cloud around, though some extensive mist and fog will form, particularly in central regions.
Sub-zero temperatures are expected to continue tomorrow morning, although things will feel slightly warmer in the afternoon.
Tomorrow will be cold to begin with, with some frost in places and also some patches of mist and fog, particularly across central parts of the country.
A largely dry day will follow, with variable cloud cover and bright or sunny spells, though Northern Ireland will be cloudy with rain later.
Wednesday will see variable cloud cover with bright spells and a few spots of rain or drizzle, though the south-west and Northern Ireland will see rain more persistent in nature.
Temperatures will plunge below 0C again on Wednesday night.
Thursday promises to be a damp day for Northern Ireland, Wales and south-west England, although it should be largely dry elsewhere.
Met Office five-day weather forecast
Today:
A few showers in the far north and east, but elsewhere it will be a dry day with a good deal of sunshine. A cold and frosty morning in the north, where fog patches may be slow to clear.
Tonight:
Cold and frosty for many areas with patchy fog, this becoming widespread across some central parts of England. A few showers for north Norfolk.
Tuesday:
Areas of fog slow to clear across central and eastern England, where it will be cold. Bright elsewhere, but cloud and wind increasing in the far west bringing rain later.
Outlook for Wednesday to Friday:
Becoming increasingly unsettled from the southwest with showers or longer spells of rain, heaviest in southwest England and south Wales. Windy at times but temperatures recovering to nearer normal.
