Millions of people have taken to the roads on what is expected to be the busiest day of the Christmas getaway.

Airports and ports are also full of people travelling to spend the festive period overseas.

A spokesman for the AA said up to 18 million cars would be on the roads over the 24-hour period from Thursday night to Friday evening.

"There will be a delayed rush hour today," he said. "Rush hour won't start until 10am as people either go off to spend Christmas away from home or do Christmas shopping, so there will be a mixture of people travelling long and short distances."

Getaway traffic is likely to be heavy on major routes, including the M1 in Buckinghamshire and Bedfordshire, the M4 between London and Newbury in Berkshire, the M25 around London, and the M4 and M5 in the West Country.

The Highways Agency reported few problems. There are delays on the A27 westbound entry slip road from the A2030 near Portsmouth due to a lorry fire, and an accident has closed one lane of the A56 northbound in Lancashire.

It said 19 sets of roadworks and lane restrictions had been removed to ease congestion, and will not be put back until the new year.

A spokewoman for Gatwick Airport said Friday is expected to be its busiest day with 100,000 passengers travelling through. Of these 58,000 were departing the UK, with a total of 1.3 million passengers going through the airport during the Christmas period.

There were also big crowds at Heathrow Airport, with 35,000 flights scheduled to arrive at Terminal 5 alone.

Some train services were delayed, with disruption reported between Gloucester and Newport in South Wales due to signalling problems, and also between Liverpool Lime Street via Runcorn or Warrington.

Unlike last year when heavy snowfall caused travel chaos, the weather is unlikely to cause any problems, with mild temperatures expected today over the whole of the UK.

A spokeswoman confirmed that the total number of passengers passing through Heathrow Airport today was around 205,700, with around 91,100 flying into the airport and another 158,400 expected tomorrow.

Helping travellers get into the Christmas spirit were helpers dressed as elves who were offering a present-wrapping service for Virgin Atlantic passengers.

A total of 4.25m Britons were expected to be travelling overseas between 16 December and 3 January, compared with just under four million for the festive period in 2010/11.

This year, around 1.7m festive travellers will be leaving from Heathrow, 425,000 from Stansted, 750,000 from Gatwick and 210,000 from Luton with another 625, 000 using Manchester, Birmingham and Glasgow airports, while many thousands will be crossing the English Channel or travelling through the Channel Tunnel.

Meanwhile, a recent AA/Populus survey of 16,647 AA members found that 68% were planning to stay at home this Christmas, with people in north east England (78%) most likely to stay put and Londoners (60%) least likely to.

More than a quarter (26%) said they would be hitting the road to spend Christmas away from home, with 10% racking up more than 100 miles.