EURO 2020 fans have basked in 25C sunshine today before Brits enjoy the “hottest day of the year” on Sunday, says the Met Office.
The mercury will soar tomorrow afternoon, just as England take on Croatia, potentially making it the warmest June 13 on record.
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Football fans will on Sunday relish the hottest day of the year so far, with temperatures hitting highs of 29C (84.2F).
Parts of the UK will be hotter than holiday destinations in Ibiza, Mykonos - and even California - due to high pressure moving in from the south.
Saturday has also been warm for England, Wales and most of Scotland, with temperatures reaching the mid-20s, the Met Office said.
London and Dorset recorded 25C by mid-afternoon, a spokesman told the Sun.
The hottest temperature recorded so far this year was 28.3C (82.94F) in Northolt, north-west London, on June 2.
Forecaster Alex Burkill said: "The picture this weekend looks very warm and sunny for much of the country, with highs of 25C (77F) in the south-east on Saturday.
"There will be some showers in parts of Northern Ireland and Scotland, and some cloud in the north-west though it will remain very warm in the sunshine."
Parts of Wales reached 21C (69.8F) on Saturday afternoon, as the nation braced itself for its first Euro 2020 match against Switzerland.
Sunday is expected to be the warmer of the two days, with the temperature possibly surpassing 30C (86F) in the South East, which has never been seen on June 13.
Mr Burkill added: "Sunday is likely to be the hottest day of the weekend, with temperatures reaching 29C (84.2F) in London and the south east, while the bulk of the country will be dry with lots of sunshine.
"This heat is likely to be widespread in the mid-20s which is above the average for this time of year."
He said that while it's "unlikely" the mercury could sneak up to 30C (86F), it "shouldn't be ruled out".
It is perfect timing as England begin their Euro 2020 campaign on Sunday afternoon against Croatia - the team that knocked them out of the World Cup three years ago.
But fans are warned UV levels will be "high or very high" and Britain's grass pollen count could cause problems for hay fever suffers.
The Met Office has issued warnings over "very high" pollen counts right across the country.
It is likely to leave sufferers with sore and itchy eyes and non-stop runny noses.
Met Office forecaster Nicola Maxey said: "Pollen rates for the whole of England and Wales are very high for the next five days.
"It's going to be quite uncomfortable for those who get it really badly.
"The UV levels are also very high over the weekend which means people could burn quite in a short space of time without realising."
As many as 13 million people suffer with hayfever in the UK.
The warmer weather marks the start of a British heatwave lasting well into next week.
John Hammond, of Weather Trending told the Sun Online: "It's certainly going to be the hottest spell of the year so far, specifically across the more southern and eastern parts of the UK.
"It’ll be quite muggy air too. This is what’s called Tropical Maritime Air, and may well carry a fair bit of cloud.
"Through Saturday and Sunday, as high pressure builds across the country, 30 degrees or more is quite possible."
The Met Office said in its that temperatures will be so high, particularly in the south, that "some areas may reach heatwave criteria".
A heatwave is defined by forecasters as an uninterrupted period of exceptionally high temperatures.
The Met Office's Oli Claydon said that the south east corner - including East Anglia and Cambridgeshire - is "where the better weather is".
And the Weather Outlook's Brian Gaze said things "will also be turning increasingly humid as a southwesterly flow pulls up sub-tropical air".
He added: "It is possible that 30C will be pipped more than once in the south between Friday and Monday.
"Nighttime temperatures will also be high in the southern half of the UK, perhaps not falling below 19C on Saturday night.
"In the north it won't be as warm, but northeastern Scotland could see temperatures into the mid 20Cs on Sunday.
"Coming after the prolonged cool period in April and May this is the weather doing a complete U turn."
The warmest day of the year so far was June 2 when the mercury soared to 28.3C in Northolt, West London.
But this weekend's temperatures may well see this record beaten.
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