The first dustings of snow will start to appear on Friday night and Saturday morning, initially spreading across the north and north Wales.
It will then spread across central and southern England and Wales. The east of England is also expected to experience freezing weather.
Snow of up to 4cm is likely across Cumbria and the Pennines and will continue throughout the weekend, with temperatures plummeting as low as -5C in rural areas, well below the December average of 3C.
There are also concerns that more heavy rain could worsen the damage already done to flood-hit areas in the South West.
The Environment Agency is warning that many rivers remain at very high levels. These include the Thames, Trent and Severn.
Around 80 flood warnings are still in place, mainly in the Midlands and the South East.
Temperatures have already dropped sharply in the past few days. On Thursday night, parts of Cumbria recorded temperatures as low as -7C, and local authorities are bracing themselves for what some fear could be Britain's coldest winter for 100 years. The Department of Health has already warned of the dangers the freezing conditions pose to the very young and elderly.
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