Heavy rain on Easter Monday could affect the return journeys of more than two million people.
The Met Office said the weather would turn from dry to cloudy with scattered rain on Sunday across much of England and Wales, with up to 15mm of persistent rain expected at the end of the Easter weekend, and possibly thunderstorms.
Some 2.01 million leisure trips by car are expected on Monday, according to figures from the RAC and transport analytics company Inrix.
They said delays were expected to be longest between 10am and 12pm and advised drivers to wait until later in the day and travel at night.
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The Met Office said a patch of North Sea cloud will spread across southeast England and push west towards Wales on Easter Sunday, causing gray weather and scattered rain in southern England, while heavy showers are possible in the south-west.
Met Office meteorologist Catherine Chalke said: “We will see more heavy rain arriving and pushing northwards on Easter Monday.
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“It’s likely to be a wet start, particularly in Wales, the Midlands and the North East, with continued heavy rain here.
“Further south, if we do see the clouds clear, we could see heavy showers again in the afternoon.”
Ms Chalke said the heaviest rainfall on Monday was likely to occur along the corridor from south-west England to East Anglia.
Sky News weather presenter Joe Wheeler said the northern half of the country will be mostly dry on Monday, with the best sunshine expected in the north-west, backing Met Office forecasts that the southern half of the country will be mostly dry Cloudy with a chance of showers or longer showers.
No weather warnings have been issued but the Met Office said showers could fall between 5mm and 15mm.
Scotland and Northern Ireland are expected to experience brighter weather on both days, with scattered showers.
The Met Office added that the outlook remains unsettled after the long weekend, with low pressure dominating and heavy rain possible.
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