Holiday traffic was already building up on the M4 on Friday morning

Safety campaigners urged drivers to take care although the RAC is reporting no major problems in the usual trouble spots on the M25, M5 and M6.

But while major roads were relatively clear, town centres were packed.

With the arrival of the colder weather, forecasters said they were certain that snow would fall somewhere in the UK on Christmas Day.

'Not as bad'

There was slow-moving traffic on the M1 northbound in Leicestershire around junction 23 and northbound at Junction 8, just north of the M25.

'The worst roads in Britain'

Town centres, including Derby, Hereford and Aylesbury, had traffic backed up with people doing last-minute shopping, while out-of-town centres like Bluewater, in Kent, and Meadowhall, in Sheffield, were quiet.

The RAC said those driving on Friday could hope for a quieter day than Thursday when accidents caused delays.

A spokeswoman said she expected a build-up of traffic in the early afternoon before things eased off.

"Very few people are going to stay at work after lunchtime and most people want to get home before it gets dark," she added.

If you start to feel sleepy at the wheel, find somewhere to stop, drink one or two cups of strong coffee or other high-caffeine drinks and take a nap of about 15 minutes RoSPA

"By about 6pm, most people will have reached their destination."

The Royal Society for the Prevention of Accidents (RoSPA) has warned that fatigue can be a problem for motorists over the festive period, particularly on long journeys after work or making Christmas preparations.

"If you start to feel sleepy at the wheel, find somewhere to stop, drink one or two cups of strong coffee or other high-caffeine drinks and take a nap of about 15 minutes," head of road safety Kevin Clinton said.

A safe journey planner can be found on the RoSPA website.

Heavy rain

The Highways Agency said driving conditions were expected to be worst on high ground, including the M6, M62, A66, and A69.

The agency added that heavy rain was forecast across most of the country on Christmas Eve, with the strongest winds expected in south-west England.

The BBC weather centre said rain would spread from the west across all parts of the UK on Thursday afternoon, perhaps turning to sleet or wet snow on the northern edge.

Wintry showers would chill those in the north and west.

Snow flakes

Millions crowded on to trains and planes on Thursday for the beginning of their Christmas breaks.

Rooftop rules on 'white Christmas'

An estimated four million passengers were travelling by train, while British Airways handled 50,000 passengers at Heathrow and Gatwick airports on Thursday, making it the busiest day of the holiday period.

A white Christmas was expected in some northern and western areas, but across the rest of Britain rain was the main problem anticipated, with winds dying down.

There was a chance the rain might turn to snow later but only by the time the busiest period on the roads had ended.