This will be followed by freezing temperatures, leaving widespread icy conditions across the UK by Sunday morning.
Much of Scotland is likely to be hit by snowfall. Up to 5cm (2in) may fall in the central belt and as much as 15cm (6in) could gather on higher ground, with only the far western coast likely to escape snow showers.
More snow will fall on Sunday, particularly in Northern Ireland, western Scotland and north-west England.
The Port of Calais is closed and passengers have been advised not to travel there. Anyone planning to make a journey is advised to contact their travel operator.
Following the closures, Kent Police warned drivers to stay away from the Folkestone and Dover areas of the M20.
The force said the severe weather conditions were making driving hazardous in this area and there was no parking for drivers who are being turned away from the Channel Tunnel.
It has brought Operation Stack into effect - which means more than 2,300 coastbound heavy goods vehicles can be parked on the M20 motorway.
Stranded driver Anniina Davie describes conditions on the A20
At Heathrow Airport, British Airways said the snow had caused "long delays throughout Friday and also some cancellations".
"Unfortunately we have had aircraft and crews out of position causing knock-on effects to our Saturday schedule," it said in a statement.
The airline advised customers to check its website - ba.com - for information on the status of their flight before travelling to the airport over the weekend.
It said customers whose flights had been cancelled could be booked on to alternative flights or could claim a full refund.
And disruption caused by snow and ice left dozens of passengers stuck on a plane at the airport for more than six hours overnight.
British Airways flight 215 to Boston, Massachusetts, was due to take off at 1600 GMT on Friday.
Very icy conditions are predicted for Sunday morning
Passengers finally boarded at 1900 GMT but found themselves still waiting to take off at 0100 GMT.
The flight was delayed for so long, crew had to be replaced because they would have exceeded their hours if they had stayed on board, said passenger Simon Wilson-Taylor.
Easyjet also said there would be further delays and cancellations. The carrier warned passengers whose flights had been cancelled on Friday that they may not be able to transfer onto flights for a few days.
It also warned against going directly to airports to make alternative arrangements because of the huge numbers of passengers already there, in particular at Luton and Gatwick.
Approximately 21 football matches have been postponed due to snow, including fixtures in the Championship, League Two and the Blue Square Premier league.
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