Temperatures plummeted to -8C (17.6F) last night with the bitter chill set to continue into tomorrow.
Experts predict the arctic freeze will take hold of Britain over the weekend as the nation prepares to shiver.
Weather warnings have been issued for this week – warning “cold does kill”.
Dr Angie Bone, of PHE’s extreme events team, said: “Cold does kill, even in places where the temperatures aren’t at their lowest.
“Now is the time to prepare for the cold weather forecast by the Met Office, and to think of those you know who are older, very young, or have pre-existing health conditions who are particularly vulnerable.”
Thursday marks the official start to winter with forecasters warning this year will be a very cold one – with snow primed to fall.
If temperatures dip much below -7C overnight into Thursday it will be the coldest start to winter since the mercury hit -15.3C in 2010.
The lowest overnight December 1 temperature since then was -7.2C at Livingston Mill, West Lothian, in 2012.
Met Office forecaster Emma Sharples said the cold snap will continue into next week.
She said: “High pressure will probably be in control of the weather until this time next week, temperatures will be lower under clear skies in the south than in the north where it will be cloudier.
“Later in the week the south and the southwest will still see widespread frosts while elsewhere they will be more patchy as cloud cover builds.
“Although temperatures will climb a little by the end of this week they will drop off again by the weekend.
“Tuesday night could beat Monday to be the coldest of autumn so far with Wednesday night looking like it will bring similar temperatures, maybe not quite as cold.”
Freak weather took hold elsewhere in the world as snow fell in the Saudi desert.
Bookies have slashed odds for a White Christmas amid predictions it could be the coldest festive season ever.
Experts have warned killer festive flu might also take hold over the Christmas season.