The mild conditions enjoyed since the start of the year are set to be replaced by snow and freezing temperatures, forecasters have warned.
Scotland and the east coast of England will face the worst of the weather, but all parts of the UK could experience snowfall.
Temperatures in 2013 have been mild so far - 10C (50F) was recorded in London on Wednesday - but the mercury is expected to plummet by the end of the week.
Sean Penston, a forecaster with MeteoGroup, the weather division of the Press Association, said: "In the next few days it will start getting colder.
"The minimum temperature in the UK will be into minus figures. By Sunday it could be minus 4 (25F) or minus 5C (23F) in some parts.
"There's more of an easterly wind coming in, with the possibility of snow showers by the weekend and the start of next week.
"It's difficult to say for certain at the moment, but in parts of Scotland there will be quite a bit of snow, as well as the east coast of England. Western areas will be a bit more sheltered from the conditions."