Drivers have been urged to take care in wet conditions as millions of festive journeys are predicted on Monday.
The AA predicts there will be about 24 million cars on UK roads on Monday, just slightly lower than the peak for the period on Friday, before festive traffic is set to be busiest on Christmas Eve.
The RAC forecasts there will be 4.2 million journeys on December 24, making it the single busiest day for getaways over the period, but with fewer commuters around roads are likely to be less busy overall.
The data also shows drivers are planning to make a further 4.2 million journeys at some point between Monday and Christmas Eve for last-minute shopping and other festive visits.
Drivers are being warned the worst hold-ups on December 24 will be between 11am and 7pm.
RAC figures suggest it could be the busiest Christmas getaway on record, with a total of 37.5 million leisure trips between Wednesday and Christmas Eve - the most in the week before Christmas Day since the company began recording the data in 2013.
Motorists battled some tricky conditions on Sunday as mist shrouded parts of the UK on the shortest day of the year.
Low pressure is set to dominate the early part of the week and the Met Office forecasts periods of heavy rain and showers on Monday in the north of England, northern Wales and parts of Scotland and Northern Ireland.
The RAC estimates the number of leisure journeys will drop on Monday and Tuesday compared with the weekend, but drivers on the Christmas getaway will be competing with commuters for road space.
Festive traffic is set to be at its heaviest on Christmas Eve when the RAC forecasts there will be 4.2 million journeys, making it the single busiest day for getaways over the period, but with fewer commuters around roads are likely to be less busy overall.
The data also shows drivers are planning to make a further 4.2 million journeys at some point between Monday and Christmas Eve for last-minute shopping and other festive visits.
Thousands of passengers heading for a Christmas getaway faced travel chaos at the weekend amid long queues towards the Port of Dover and major train cancellations.
Ferry passengers were advised to “not arrive any earlier than two hours before their scheduled sailing”, as delays were experienced on the approach to the Kent port on Sunday morning.
Nearly 30,000 cars are estimated to embark on outbound sailings during the festive period.
Peak traffic at the port will be between 6am and 1pm, when P&O Ferries, which operates some sailings , urged customers to allow extra time for their journey.
Doug Bannister, chief executive of the Port of Dover, said: "To help ensure a smooth departure, we kindly ask all customers travelling with our ferry partners not to arrive any earlier than two hours before their scheduled sailing, just as you would if departing from a UK airport, and greatly appreciate everyone's patience should there be any additional waiting time due to the high volumes of traffic."
Meanwhile, rail passengers planning to travel on Cross Country trains faced cancellations on Sunday amid a shortage of drivers.
The operator, which runs long-distance inner city-trains, warned customers that services were set to be busier than normal.
Cross Country, which operates long-distance inter-city trains, warned passengers that services would be busier than usual at the weekend, with a number of services between Manchester and Bournemouth cancelled on Sunday.
Southwestern Railway customers have also been advised to check before they travel over the Christmas and New Year period, due to seasonal changes and major engineering work between Clapham Junction and London Waterloo.
It will see an amended timetable on Christmas Eve, with services finishing earlier than usual, at around 2200. Island Line services will also finish earlier than usual and there will be no SWR services on Christmas Day and Boxing Day.
Whilst other SWR services on the network will resume, on Saturday 27 and Sunday 28 December there will be no services between London Waterloo and Clapham Junction. An amended timetable will operate on all lines via Barnes and Wimbledon, with services starting and finishing their journeys at Clapham Junction, the operator said.
Services will also be amended between Boxing Day and January 4.
Stuart Meek, chief operating officer of South Western Railway and Network Rail (Wessex) said: “There will be major changes through to the new year, particularly for services to and from London Waterloo via Barnes, and on Saturday 27 and Sunday 28 December, when there are no trains between London Waterloo and Clapham Junction.
“Tens of millions of customers travel on our lines between London Waterloo and Clapham Junction every year, and once this vital engineering work is complete, they can expect a more reliable railway and smoother journeys for years to come.”
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