Wintry conditions are affecting large parts of the UK, with snow and ice and heavy rain from northern Scotland to the southern counties of England.
Met Office yellow “be aware” warnings are in place for Scotland, north Wales, most of England and Northern Ireland.
Temperatures fell sharply on Saturday night, with a low of -13.5C (7.7F) in the Highlands village of Dalwhinnie.
In north Devon, several homes have been evacuated after “widespread flooding” sparked by heavy rain.
The Met Office warning of snow and ice is valid from 06:00 GMT until 18:00 GMT and predicts snow on higher routes and says rain may fall onto frozen surfaces for a time, particularly across Scotland.
Saturday night was the coldest since -14.1C was recorded at Braemar in Aberdeenshire on 14 February 2016.
The alert, originally issued on Saturday, was later extended northwards to cover the northwest Highlands, parts of the Western Isles and southwards to cover more of Hertfordshire, Essex and Suffolk.
The Met Office chief forecaster said the amounts of fresh snow would vary considerably across the warning area, with 3-6 cm possible on higher ground and 1-3 cm at lower levels, although some spots would not see any.
Travel has been disrupted, with flights in and out of London Stansted Airport affected.
East Midlands Airport temporarily grounded planes due to snowfall, while Glasgow Airport reopened after it was closed for a short time for the runway to be cleared of snow.
Drivers in south-west Scotland were urged to “avoid unnecessary journeys”.
Five people were injured in a road accident close to Glencoe Mountain Resort ski centre.
Police asked drivers to avoid the A82 around the resort, where drifting snow and white-out conditions were described as “hazardous”.
Heavy snowfall across Dumfries and Galloway was causing problems on roads including the A75, which was blocked just outside Dumfries for a time.
Meanwhile, two climbers were airlifted to safety after being discovered in a “precarious position” on a mountain ridge in Glencoe since Friday.
The pair had survived a night without shelter in sub-zero temperatures after becoming stuck at 3,000 ft at Bidean nam Bian.
Blizzard conditions had prevented a mountain rescue team from reaching them.
In Wales, Met Office warnings are in place for Wrexham, Conwy, Denbighshire, Gwynedd, Powys and Flintshire. Up to 6cm (2.3in) of snow is forecast to fall on ground above 200m (656ft).
Snow is affecting driving conditions on the A470 between Betws-y-coed and Blaenau Ffestiniog. The M4 motorway was partially closed near Bridgend after a serious accident involving an overturned car.
South Wales Police urged motorists to avoid travelling, saying driving conditions were hazardous with spray and standing water on the road.
A Met Office warning of rain is in place for central and southern Wales and north Devon.
Police in Devon said properties in Kentisbury, near Barnstaple, and Combe Martin on nearby Exmoor, have been affected by the conditions.
The A399 at Combe Martin has been closed following a landslide, while the A361 is also shut near Landkey in Barnstaple.
Devon County Council told residents unable to stay at home due to flooding to ask for emergency accommodation, advising people to “take great care”.
There are no warnings in force for Monday, when forecasters say rain will clear away from southern areas and northern Scotland to leave a breezy but fine afternoon.
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