IT'S officially the hottest day of the year so far after the mercury hit a scorching 27.8C - and the weather will only get warmer as the week goes on.
But while millions of Brits are making the most of the mini heatwave, health chiefs have issued a slew of warnings as the temperature rockets on Thursday and Friday.
Forecasters are predicting baking highs of 34C on what is being labelled "Fiery Friday".
It means the temperature record for the year so far could be broken for three days in a row after forecasters confirmed a new high was reached in Heathrow, London today.
The simmering temperatures - caused by high pressure and roasting 40C air flowing up from Spain and Portugal - have forced the Met Office and the UK Health Security Agency (UKHSA) to issue heat-health alerts.
A level two warning - called alert and readiness - has been put in place for a large part of southern and central England.
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These are triggered when there is a 60 percent risk that temperature thresholds will be reached in one or more regions on at least two consecutive days and the intervening night.
The heatwave temperature threshold varies in each county - in the UK it is between 25C and 28C.
While a level one alert - the lowest warning which calls for a minimum state of vigilance - is in place for northern England.
Agostinho Sousa, Head of Extreme Events and Health Protection at the UK Health Security Agency, told Brits to look out for the vulnerable as the mercury rises.
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He said: “During periods of hot weather it is especially important to keep checking on those who are most vulnerable, such as older people and those with heart or lung conditions.
"Make sure to look out for signs of heat exhaustion and follow our simple health advice to beat the heat.”
Meanwhile Greg Dewhurst, a Met Office forecaster, told The Mirror Wednesday will be a "lovely day" for much of England and Wales.
But it's bad news for Scotland and Northern Ireland which will see "cloud and showers".
He went on: “It will be very warm in the South East in particular, with the temperature hitting a high of 28C, which would make it the warmest day of the year so far.
“This will then be surpassed on Thursday, with the mercury rising again to around 29C before reaching 33C and even 34C on Friday.
“So it’s going to be very warm as we move into the later part of the week.”
The BBC said Wednesday evening will see cloud clear for most although Northern Ireland and the far north and west of Scotland will remain under the cloud.
Showers in the far north and west are forecast to ease on Thursday morning.
Parts of the UK will be hotter than summer holiday destinations like the Greek Islands of Santorini, Mykonos and Zakynthos.
And Britain will even tip Los Angeles in the heat race on Wednesday.
Met Office Deputy Chief Meteorologist, Dan Rudman, noted it is "still unusual for temperature to exceed these values in June".
The highest temperature reached in the UK so far this year is 27.5C at Heathrow on May 17.
And you have to wind back the clocks to 1976 for the hottest June date on record, which saw Southampton Mayflower Park sizzle to 35.6C on June 28 that year.
First aid and health response charity St John Ambulance has issued some tips to handle the hot weather.
Its medical director Dr Lynn Thomas said: "If you're out and about in the sunshine, it's important to make sure you look after yourself by staying hydrated, keeping out of the sun at peak times, and by wearing sunscreen with a minimum SPF 30.
"I would also encourage anyone with elderly relatives and neighbours to check in with them, as any increase in temperature can be dangerous."
Meanwhile charity Age UK is urging elderly people to take some simple precautions, particularly if they have breathing problems or a heart condition.
It is still unusual for temperature to exceed these values in June
Dan Rudman, Met Office Deputy Chief Meteorologist
Caroline Abrahams, of the charity, said: "Older people can be at risk of dehydration and overheating when it gets hot, especially if they live somewhere that is hard to keep cool.
"So it's a good idea to let people know if you have any concerns about yourself, especially if you live alone.
"For the rest of us, checking in on older relatives and neighbours is a nice idea - for example, you may have an extra fan you can lend, and the offer of an ice cream when it is sweltering will usually be appreciated too.
"Taking simple steps to keep cool during the hottest parts of the day is a good idea."
She advised elderly people to remain indoors, wear light clothing and drink plenty of water during the warmest times in the day.
The UK's heat health warnings are split into four levels on an alert scale - designed to help health workers negotiate and prepare for heatwaves.
According to the Met Office, level Zero sees long-term planning to reduce risk from heatwaves and includes year-round joint working to reduce the impact of climate change.
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While level one and two is what the UK will experience on Wednesday.
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Level three is issued when the Heat-Health Alert thresholds have been exceeded and level four is rolled out when a prolonged hot spell becomes severe.