Millions are bracing themselves for what is likely to be one of the busiest travel days of the year as people gear up for Christmas.
The AA is expecting widespread congestion around shopping centres, football venues and ports.
Queues of around 90 minutes have been reported at the port of Dover.
It is also be the last chance for people to get trains to or from London Paddington and Kings Cross, which are closed from Christmas Eve.
However, road congestion is likely to have peaked on Friday afternoon as drivers embarking on leisure trips competed for road space with commuters and business traffic.
Some of the busiest areas on Saturday are expected to be around London's Heathrow Airport, Manchester's Trafford Centre and Avonmouth, the AA said.
"AA advice is to plan your journey and be prepared for delays," a spokesman said.
The RAC estimated there would be 13.5 million leisure journeys by car across the UK between Friday and Sunday, up 20% on the three days before Christmas Day last year.
High streets up and down the country are expected to be very busy, with many people appearing to leave their Christmas shopping late this year, the British Retail Consortium said.
"Our expectation is that this weekend will be particularly busy on Britain's high streets, possibly the busiest trading weekend of the year, with a strong focus on shopping for last minute gifts," a spokesman said.
For those travelling by sea, Port of Dover Travel posted on social media: "There is currently a 90-minute wait time ahead of French border controls as we help passengers get away for Christmas."
P&O Ferries apologised for delays, telling customers: "We are working hard with the authorities involved to improve the situation and would like to assure you that you will be accommodated on the next available sailing once you get through."
The Met Office has warned that poor weather conditions could also cause travel issues in parts of the UK over the weekend.
High winds are due to hit the north of Scotland and northern and central areas of England on Christmas Eve.
Two separate yellow wind warnings have been issued, saying people should expect travel disruption, damage to buildings and power cuts, while the rest of the country may see showers.
A yellow warning for rain covers much of Wales, with forecasters warning flooding and travel disruption is possible.
Network Rail warned severe weather was likely to impact some ScotRail routes on Saturday.
Widespread rain is expected throughout the UK on Christmas Day.
