The Met Office is warning that temperatures will fall to their lowest levels of winter so far this week, reaching as low as minus one degree on Thursday.
The weather service this morning issued a yellow alert for snow for London and South East England including Buckinghamshire, Milton Keynes, Oxfordshire, Hampshire & West Berkshire.
Sleet and light snow is predicted to fall from about 6am, although it is not expected to be heavy. Snow is most likely on higher ground.
Met Office forecaster Mark Wilson said people should wrap up warm, noting that freezing temperatures could be a risk of ice.
That's far from the lowest levels in the country - last night temperatures fell as low as -8.3*C in Braemar, Aberdeenshire. And Northern Ireland experienced its coldest night of the winter so far, with a temperature of -5.2*C recorded at Katesbridge.
But it's not all bad news: temperatures are expected to pick up towards the end of the week.
"Snow may become rather more of a problem prior to eventual clearance on Wednesday morning," the Met Office said. "There is some uncertainty both over the clearance time, as well as the degree to which rain turns to snow, so this warning may well be updated nearer the time."