British drivers travelling to see friends and family in the run-up to Christmas Day will be estimated to take a total of 20 million trips.
Leisure traffic will build steadily from Monday before peaking on Friday and Saturday, which is Christmas Eve, the RAC said.
A strike by thousands of members of the Rail, Maritime and Transport union at Network Rail from 6pm on Christmas Eve will increase pressure on the roads.
Train passengers are being urged to complete journeys as early as lunchtime that day, meaning many people will opt for car or coach travel instead.
The RAC predicted 7.9 million journeys will be made over the two days immediately before Christmas.
The worst congestion on those days is expected to be between 10am and 7pm on Friday, and between noon and 1pm on Saturday.
Total traffic will be far greater before the weekend as drivers getting away for the festive period compete for road space with commuters and everyday motorists.
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This is likely to cause jams in cities and on major routes.
Transport analytics company Inrix expects journey times to take about 14 per cent more time than the same period last year.
Roads likely to be hit by congestion this week include the M25, the M60 near Manchester, the M6 in north-west England and the M40 in Oxfordshire.
National Highways said it would ensure almost 98 per cent of England’s motorways and major A-roads are fully open from 6am on Tuesday until the end of January 2, by either completing or lifting roadworks.
“Our research suggests getaway traffic will build steadily all week, culminating in two frantic days of travelling just before Christmas," RAC spokesman Rod Dennis said.
“By Friday, we fear it will be a case of jams today and jams tomorrow on the roads as millions of people jump behind the wheel to see family and friends, with last-minute Christmas shoppers as well as the impact of another rail strike also leading to an increase in traffic.
“If possible, drivers should try to make their trips on a quieter day earlier this week if they possibly can.”
The RAC’s traffic estimates are based on a survey of 1,886 UK adults conducted last week.
The Automobile Association, which issued an “amber traffic warning” for Friday and Saturday, urged motorists to carry out car checks, such as levels of fuel, oil, coolant and screen wash, and ensure tyres are correctly inflated.
"Breakdowns add to traffic jams, so if everyone spent 10 minutes checking their car before leaving it could save everyone hours on their journey," said AA’s head of roads policy, Jack Cousens.
