The great Christmas getaway has started with millions hitting roads and airports ahead of the holiday period.
The fog has caused chaos at Heathrow airport
But thousands of air passengers are facing problems after fog forced Heathrow to cancel 350 flights. Some regional airports are also affected.
Congestion began to build up soon after lunchtime and widespread traffic jams were reported on many major routes.
Weather forecasters, predicting more fog and frost, have urged motorists to take extra care.
Road information company Trafficmaster predicts more than 18 million vehicles will be on the road this weekend.
AA FOG ADVICE
Use fog lights when appropriate
Do not keep switching fog lights on and off - this can be a distraction
Only drive as fast as conditions allow
Maintain a greater distance between you and the car in front
HGVs making last-minute deliveries, commuters leaving work early for holidays and last-minute Christmas shoppers are expected to add to the congestion.
Friday is also expected to be a busy day for travellers.
The Highways Agency said 26 sets of roadworks in England are to be suspended over the Christmas and New Year period.
The roadworks will be removed from 0600GMT on Friday, until 0100GMT on 2 January.
However, a total of 35 roadworks will remain in place for safety reasons.
They include the M1 at junction 6a to 10; the A38 at Streethay, West Midlands; the M6 junction 9-8 southbound; the A66 at Surtees Bridge, Stockton-on-Tees; the M62 at junction 21 and junction 25 on the M25.
The weather conditions have led British Airways (BA) to cancel all domestic flights from Heathrow and many European ones.
Check with your airline
Do not go to the airport if your flight is cancelled
Do not bring family and friends to the airport
Take only one item of hand luggage
Do not take wrapped presents through airport security
Pack and prepare your luggage before leaving home
Liquids must be contained in individual containers of no more than 100mls
Liquid bottles must be contained in a transparent, re-sealable plastic bag no larger than 20cm x 20cm
Liquids in containers of more than 100ml capacity can be packed in your checked-in baggage
British Airways helpline 0800727800
Flights from Coventry were transferred to other airports and some from Cardiff have been cancelled, while some BA flights from Scottish airports have also been scrapped.
Sleeping mats, blankets, catering facilities and hot drinks are being provided for stranded passengers at Heathrow, but there are fears that some people will not get a flight before Christmas.
The Association of Train Operating Companies (ATOC) said there is likely to be an increase in demand for train journeys as more flights are cancelled.
Virgin Trains are expected to be running more trains over the next couple of days, including an extra seven services on Friday.
An ATOC spokesman said they expected an increase in passengers travelling long distance routes - including London to Edinburgh, Manchester and Newcastle - and urged people to make reservations if possible.
"We would suggest people avoid the peak period between 1430GMT and 1930GMT if they don't have a reservation as it is going to be very busy today and Friday," he said.
Air passengers have been forced to sleep in airport terminals
A spokesman for Eurostar said it had seen a 15% increase in passengers between London and Paris and Brussels since Wednesday lunchtime.
He urged passengers to check the Eurostar website for availability and to order tickets online or by telephone before arriving at Waterloo.
Barry Gromett of the Met Office offered a glimmer of hope for stranded air passengers.
"There is a cloud sheet edging its way westward across Kent and Essex and clearing the fog, which should help to alleviate some problems at airports today.
"But we are not out of the wood yet and if that cloud cover breaks then the conditions are right for fog to reform."
Mr Gromett added that motorists could expect fog, frost and generally cold but dry conditions across England and Wales, with winds and warmer weather in Scotland.
Matt Taylor from the BBC Weather Centre said there was unlikely to be snow this year.
"I think it is going to be a grey Christmas for many, instead of a white one."