The last time the mercury plummeted this low was in March - at the summit of a Scottish mountain.
The north and the east of the country are in line for heavy snow, although forecasters say this could spread across Britain as we head to the weekend.
For the start of the week temperatures will be average, with daytime highs of seven or eight Celsius.
But as December nears this will fall, with a north easterly wind battering the coast and bringing showers with it.
Rural parts of the country could see overnight temperatures as low a -10C, said Met Office forecast Alex Fox.
He said: 'The last time temperatures reached that low was on the night of the 9th and 10th of March this year at the top of Braemar, a mountain in Scotland that's 1,000ft above sea level.
That gives us an idea of how cold things are going to get.
'With these conditions there's an increased chance we could see snow, especially in northern and eastern parts of the country.
'And some places could get a significant amount of snow as we head towards the end of week.'
'We are getting closer and closer to December in that transition phase. At the moment temperatures are about average but over the next few days this will definitely get colder.
'We will still see showers, although not heavy rain, on Monday and Tuesday in some parts of the country.
'But by the end of the week this could have turned to snow hitting the south by late Wednesday, early Thursday and staying until the weekend.'