Hottest early May Bank Holiday Monday expected
Temperatures could reach 27C (81F) in parts of England on Monday, which would make it the hottest early May Bank Holiday on record.
The South East can expect the highest temperatures - while Wales and Scotland could also see temperatures in the mid to high 20s, BBC forecasters said.
The warmest early May Bank Holiday Monday on record was 23.6C, in 1999.
But this Monday could be the hottest since 1978, when the early May Bank Holiday was introduced.
The average high for the May Bank Holiday in London is about 18C.
The highest temperatures of 26C and 27C are expected in south-east England, particularly around west London.
In East Anglia temperatures could reach 25C, while northern England and Wales are likely to have highs of 23C.
It will be slightly cooler in south-west England, Scotland and Northern Ireland, with most places between 19C to 22C.
The RAC said a total of 8.5 million journeys are expected on the road between Friday and Monday.
By 17:00 BST on Friday, stretches of several motorways were heavily congested, the RAC said. Traffic was at a standstill at the M5 and M4 near Bristol and there were long delays on roads near Exeter, as people headed to the south-west of England.
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"Bank Holiday Monday looks like it will be a fine and dry day for almost all of the UK with plenty of blue sky and warm sunshine," said BBC Weather's Nikki Berry.
"Although many may head to the beach to soak up the sun, it is likely to be a few degrees cooler on the coast as sea breezes develop.
"Sea fog may also plague some North Sea coasts in Scotland."
It comes in sharp contrast to last Monday when some parts of the UK experienced "unseasonably cold weather" and saw more than half a month's rainfall in a day.
And in early April, parts of Scotland, northern England and north Wales were covered in heavy snow.
The highest May temperature ever recorded in the UK was on 29 May 1944, when Regent's Park, Horsham and Tunbridge Wells reached 32.8C (91F).