'No let up' - The people working in extreme heat
ByElla Kipling and Anna Lamche
The UK has experienced its hottest June day on record after temperatures soared to 36.1C (97F) in Gosport, Hampshire on Wednesday afternoon.
Hundreds of schools shut across England and Wales and transport has also been disrupted, with train passengers advised to avoid all non-essential travel.
The heatwave is forecast to continue into Thursday and Friday, with a further high of about 38C possible, the Met Office said.
A red extreme heat warning issued by the Met Office across parts of south and central England and south Wales remains in place until 23:59 BST on Thursday.
The high for the day came between 15:00 and 16:00 BST - breaking the previous June record of 35.6C recorded in Southampton in 1976 and Camden in 1957.
This new record is described as "provisional" by the Met Office, which now has to conduct checks to ensure the measurement is reliable.
The next highest temperatures of the day were all recorded in southern England, including Wisley, Surrey on 36C, Wiggonholt, West Sussex on 35.9C, and Charlwood. Surrey on 35.7C. And Wales had its hottest day of the year so far, with a high of 33.3C at Cardiff Bute Park.
Meanwhile, a searing European heatwave continues to cause deaths and disruption, - with France recording its hottest day since records began in 1947, and the temperature in Paris and other areas above 40C.
Among the areas covered by the UK red warning are the East Midlands, the east of England, London and south-east England, south-west England, Wales and the West Midlands.
These regions can expect an "exceptional spell of hot and humid weather" with impacts to the general population "highly likely", according to the Met Office.
It is only the second time a red warning has been issued since extreme heat warnings began in 2021.
June temperature records set in 1976 and 1957 were broken
A wider amber warning covers much of England and Wales through Thursday,, external with areas as far north as Manchester expected to see temperatures reach above 30C.
A separate amber warning for extreme heat, lasting from midnight on Friday, external until 23:59 covers south-east England, eastern England and parts of the Midlands. And a new amber weather warning has been issued, external for between 00:00 and 21:00 on Saturday, covering parts of southern and eastern England.
The daily air temperature record for the UK in June is 35.6C, which was reached on 28 June 1976 - but this is very likely to be exceeded in the coming days.
The "feels like" temperature, which indicates how people will experience the heat, is likely to reach more than 40C in parts of England.
King Charles got a helping hand to stay cool when he attended a London Climate Week reception
Matthew Lehnert, chief forecaster at the Met Office, said high humidity means the heat will feel "much more potent".
Parts of the UK are also set to experience "tropical nights", which occur when night-time temperatures stay above 20C.
The Met Office has warned that the weather could result in "population-wide adverse health effects", leading to serious illness or danger to life.
Changes in working practices will be required, and there will be "significantly more" people visiting coastal areas and lakes, leading to an increased risk of water safety incidents.
It comes as a major rescue operation was initiated in Hampshire after a 15-year-old boy went missing while swimming in a nature reserve lake.
Several people stranded in traffic on the M25 near Godstone in Surrey after a crash are being treated for heat-related illnesses, South East Coast Ambulance Service said.
Train operators including South Western Railway, Thameslink, Northern and Avanti West Coast have asked people to only make essential journeys and have put reduced timetables in place. Transport for London warned passengers that "very high temperatures may cause disruption to some Tube and rail services.
Ice cream man Emery, 17, said customers were coming out on "almost every street" because of the heat
The heat has affected working people across the country. Telecoms engineer Jake Bird told the BBC how he was preparing for a week of high temperatures in his job, which requires him to climb telephone poles in PPE and a harness.
The 21-year-old has decided to keep working as he is self-employed but said he would take strategic cooling breaks.
The engineer, from Oxfordshire, said: "Here it's going to be 36 degrees and that is just insane, especially going up a pole in all that kit and everything. It's just going to be absolutely dreadful."
Meanwhile, bricklayer Elijah told BBC Newsbeat he tries to "keep the complaining [about the heat] to a minimum because about 10 months of the year, you're in rain and the cold".
But he admitted the heat is "a lot to deal with", so his team in London has been starting work earlier than normal and finishing by midday. He also gives his employees water and ice lollies to keep them cool.
For 17-year-old ice cream man Emery, from Gloucester, the heat is "good for money and for the business". However, the extreme heat also brings costs: everything in the van has to be plugged in overnight to keep it cool for the next day.
The UK Health Security Agency (UKHSA) has also issued a red heat-health alert, which remains in place until 23:00 BST on Thursday.
Meanwhile, Network Rail is advising passengers travelling to, from or within the current red weather warning zone to only to make necessary journeys on Wednesday, Thursday and Friday.
Dr Agostinho Sousa, head of extreme events and health protection at UKHSA, said it was "vitally important" for people to understand the risk the high temperatures poses.
"A red heat health alert indicates a risk to life for even the healthy population, but simple actions like staying hydrated, avoiding the sun during the hottest part of the day, and keeping your home cool can make a big difference," he said.
Transport for London warned passengers that "very high temperatures may cause disruption to some services
More than 800 schools in England have been disrupted because of the high temperatures. The Department for Education said on Monday that it does not normally advise schools to shut during hot weather.
It said that attendance was "the best way for pupils to learn" and that hot weather could "usually be managed safely".
Emma Howard-Boyd from the National Heat Commission said the closure of schools and transport routes showed that the UK was "not prepared" for the heatwave.
She told BBC Breakfast the country had to prepare for the heat "we know is coming in the months and years to come".
Scientists have warned hotter temperatures are likely in the years ahead. Between 2015 and 2024, the number of days exceeding 30C in the UK more than trebled compared with the 1961-1990 average, the Met Office says.
Howard-Boyd said: "We need to make sure that as we're upgrading, refurbishing, rebuilding our schools, that we have heat at the top of the agenda. It's the same for our transport networks."
Northern Ireland and Scotland experienced their hottest days of the year on Tuesday, with temperatures reaching 28.1C in Katesbridge, County Down and 29C in Dyce in Aberdeen respectively.