Met Office forecasters have warned parts of the UK face snow and ice as temperatures drop as low as -10C.
Large parts of Scotland and areas of northern England, including the Pennines and the Yorkshire Moors, are predicted to see snow and icy conditions – which could cause travel disruptions.
Temperatures may drop as low as -7C (19F) overnight on Wednesday in Aberdeenshire, eastern Scotland.
Parts of the south will also begin to feel the winter chill, with temperatures dipping down to -1C (30F) in south-west London.
But the coldest weather is expected in western Scotland overnight on Thursday, where temperatures could drop to an icy -10C.
The Met Office said the cold spell is expected to last until Friday, but that the weekend would see brighter weather and even some sunshine.
It added that temperatures going into next week would be lower than the average annual temperature, which is usually around 7-9C (45-48F).
Chief meteorologist Dan Suri said: “It’ll certainly feel more wintry this week, with colder weather bringing a risk of frost, ice and wintry showers.
“The risk of snow accumulating is largely confined to the northern half of the UK – mainly over higher ground in Scotland, Wales and northern England.
“By Thursday morning parts of northwest Scotland could see 2cm of snow accumulations to low levels, with 2-5cm above 200m and up to 10cm gathering over the highest routes, leading to some travel disruption.”
The Met has put in place snow and ice warnings for parts of Scotland from 6pm this evening until 12pm on Thursday, along with an ice warning for Northern Ireland on Wednesday night.
Traffic Scotland operator manager Douglas Cairns said that a record number of gritters had been made available to keep traffic moving this winter.
“The first severe weather warning of the winter for snow and ice is always a timely reminder for people to check they are winter ready and have made appropriate preparations,” he said.
But deputy chief meteorologist, Dan Harris, said that despite the cold and unsettled weather – including freezing fog – all was not “doom and gloom.”
“We’re keeping an eye on other areas where there is potential for disruptive snow – in particular central and south-east England and the hills of northern England,” he said.
“It’ll stay cold into the weekend with rain, showers and hill snow continuing across many areas, along with overnight frosts.
“We could also see some stubborn areas of freezing fog develop more widely, which at this time of year can persist all day.
“However, it’s not all doom and gloom, as there’s likely to be an increased chance of dry, bright, and perhaps even sunny weather on offer as the weekend progresses.”