Heavy rain could disrupt more than two million return journeys on Easter Monday.
The Met Office has said drier spells will turn cloudy with patchy rain for much of England and Wales on Sunday, before up to 15mm of more persistent and possibly thundery rain at the end of the Easter weekend.
Some 2.01 million leisure journeys are expected to be made by car on Monday, according to the RAC and transport analysis company Inrix.
They have said the lengthiest delays are expected between 10am and 12pm, and advised drivers to wait until later in the day and travel in the evening.
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An area of cloud in the North Sea will spill over into South East England and push west towards Wales on Easter Sunday, the Met Office said, leading to grey conditions and patchy rain across southern England, while the South West could see heavy showers.
Kathryn Chalk, meteorologist at the Met Office, said: “We’ll see further spells of heavy rain coming in, pushing its way northwards through Easter Monday.
“Probably a damp start, especially across Wales, central England and northeastern parts as well with further heavy spells of rain here.
“Further towards the south, if we do see any clouds breaking up we could see heavy showers again in the afternoon.”
The heaviest of the rain on Monday will likely be in a corridor from South West England up to East Anglia, Ms Chalk said.
Sky News weather presenter Jo Wheeler says northern parts of the country will be mostly dry with the best of the sunshine in the North West on Monday and backs up the Met Office’s prediction that the southern half of the country will be largely cloudy with showers or longer spells of rain.
No weather warnings have been issued, but the Met Office said the showers could see between 5mm and 15mm of rainfall.
Scotland and Northern Ireland are expected to hold on to much brighter spells with odd, scattered showers across both of the days.
The outlook will remain unsettled after the long weekend, with low pressure dominating and spells of heavy rain likely, the Met Office added.