Temperatures have hit 34.7C in parts of England, as temperatures soar across the UK - see the warmest areas as of 14:00 BST
It comes as a red warning for extreme heat in parts of England and Wales, lasting until 23:59 BST on Thursday, has been extended to cover more of southern England - red alerts explained
Rail passengers are being warned to avoid non-essential travel, with a new amber heat warning issued for Saturday in southern and eastern England
Meanwhile more than 1,000 schools in England and Wales are shut or closing early
Johnny, a construction worker in London, says the heat is "horrible" but "there's work to be done" - while in Shetland, Helen tells the BBC it's woolly dress and vest weather
Fans, loose clothing, lots of water - follow these tips to stay cool
The highest temperatures recorded so far today have been at Bournemouth Airport and Thorney Island in West Sussex, where it was 34.7C as of 14:00.
That tops yesterday, when the maximum temperature in England was 34.6C - and it's going to get hotter still.
Forecasters says the UK’s June temperature record, which stands at 35.6C, could be broken today.
Rowan Bridge
North of England reporter
Image source, Getty ImagesThe North Yorkshire Moors Railway, which operates steam trains, says it’s switching to diesel locomotives on parts of its route because of the weather.
The volunteer-run railway says it’s taken the decision because of the increased risk of triggering fires by the line in the heatwave.
It says problems could be caused by sparks coming out of the train’s chimneys potentially starting fires.
It’s switched to diesel for the route which runs between Pickering and Grosmont through the North Yorkshire Moors.
It’s still running steam trains in the areas at lower risk between Grosmont and Whitby, but says it’s monitoring the situation.
Image source, EPA/ShutterstockA man readies himself - with a little help - for the sun's rays at Margate
Image source, ReutersPupils at Harris Academy Primary Mayflower school are keeping cool during a maths lesson with ice lollies
Image source, Getty ImagesPeople are looking for all manner of way to find some shade
Image source, Getty ImagesAnd what a day to be punting on the River Cam in Cambridge
Yasmin Malik
BBC Newsbeat
Image source, Jake BirdNo working from home for engineer Jake
Telecoms engineer Jake Bird is among those who can't avoid the sun when he's at work.
The 21-year-old from Oxfordshire spends his working day climbing up telephone poles in full PPE and a harness.
He says he plans to keep working as he's self-employed but will have to take strategic cooling breaks.
Jake adds: "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."
Image source, Getty ImagesThe majority of Eurostar's trains are continuing to operate on both cross-Channel and continental routes, but the operator advises vulnerable passengers to postpone their journey "where possible".
A spokesperson for the high speed rail operator says it has a small number of cancellations due to extreme heat conditions affecting rail infrastructure.
High speed trains between Brussels and the French border will "run at reduced speeds", at the request of Belgium's infrastructure network operator, the spokesperson says.
This will not affect London-Paris trains, and the company adds that Eurostar customers can change their journey for free if they wish to travel outside the heatwave.
Hazel Shearing
Education correspondent
We've now counted 837 schools in England which have fully or partially closed, or which are offering parents the option to pick their children up early.
Most schools are open, but that doesn't mean all their pupils are present. Some trusts have reported a big impact on attendance.
Education Secretary Bridget Phillipson is warning that pupils in England shouldn't be missing school when they can go in because they'll miss out on learning.
"I know hot weather can be a struggle. But my message to families is simple: if your child’s school is open, you should keep sending them into school," she says.
“Every day of absence and lesson missed has a cost – and that cost falls hardest on our most disadvantaged pupils and working class communities.”
She adds that relaxed uniform rules are among the things schools are doing to keep children cool, and any family with concerns about medical needs should talk to their school.
Hundreds of schools are also fully or partially closed in Wales.
Temperatures in England reached 33.9C at 13:00 this afternoon, recorded at Thorney Island in West Sussex.
Cavendish in Suffolk was 33.6C at the same time, while both Wisley in Surrey and Shoreham in West Sussex reached 33.4C. A maximum of 39C is not out of the question for southern England on Wednesday.
It was 30.3C in Cardiff, Wales. In Scotland, the 13:00 temperature was recorded as 24.2C in Charterhall. In Northern Ireland, it reached 23.4C at Castlederg.
Image source, PA MediaLetters and parcels will continue to be delivered as normal through the heatwave, Royal Mail says.
But the company says measures are in place to help manage the severe heat for postal workers.
It says local teams will review their plans and "adjust working arrangements where needed" to avoid over-burdening staff during the hottest parts of the day.
Manish Pandey
Live reporter
Image source, EPA/ShutterstockTemperatures have continued to rise throughout the day, with highs of 37 or 38C expected to break the record for June later.
The Met Office's red warning for extreme heat is in effect and has been extended to 23:59 BST on Thursday.
The East Midlands, East of England, London & South East England, South West England, Wales and the West Midlands are all included - read more on what an extreme heat warning means for you.
High humidity will make it feel particularly hot when combined with the strong sunshine, says our forecaster.
The heat has caused significant disruption, with over 1,000 schools fully or partially closed in England and Wales.
Transport has also been hit, with rail operators altering their services and National Rail warning passengers not to travel unless absolutely necessary.
Meanwhile, in Europe the heatwave continues, with half of France remaining under a red heat alert.
Peter Gillibrand
BBC Newsbeat
Image source, SuppliedElijah brings in water and ice lollies to help keep his team cool
Bricklayer Elijah knows a thing or two about working in the elements.
He spends every working day outside and tells BBC Newsbeat that getting through bad weather, be it cold or heat, is just “part of the job”.
“I tried to keep the complaining to a minimum because about ten months of the year, you're in rain and the cold,” he says.
He admits the heat has been “a lot to deal with”, adding: “We've been starting early, at like 06:30 or 07:00, and trying to get out by midday because by [that time], there's not much you can really do at that point.”
He's laying on water and ice lollies to help keep his employees cool but says “I tell people if they don't want to come in because of the heat don't bother, don't risk it".
Esyllt Carr
Business reporter, BBC News
Image source, Getty ImagesGreggs has temporarily closed 11 stores today and tomorrow for the safety of staff and customers.
Staff at one London branch say employees have been moved from one shuttered store to others around capital, telling the BBC: “It's very hot. We have industrial ovens but no proper air conditioning in stores.”
The stores affected are: Aldridge, Wolverhampton, Luton, Gosport, Trowbridge, Barnet, Marlborough, Wantage, Dudley, Northampton and Leicester Square, London.
Jasmine Sandhar
BBC Newsbeat
The heat's been good for Emery's business
Emery, 17 from Gloucester, is having a good afternoon.
The ice cream man is out with his dad and says customers are coming out on "almost every street" during their rounds, with the heat being “good for money and for the business".
But there are costs too - everything in the van is plugged in overnight to keep it cool for the next day.
During scorching days, they make sure to keep the windows open and have a vent in place to stop their 99s and lollies becoming puddles.
The family also run a burger van but that’s taking a back seat this week. Emery says: "You have the air fryers coming up for 120 degrees. My co-worker’s shirt was drenched in sweat."
Image source, Getty ImagesOver on BBC Radio 5 Live, chef Luke in Colchester says the heat is "an occupational hazard" and they "have to get on with it" adding: "We have hot lamps that are running from midday to midnight.
"Ovens running at 200 degrees all day. Gas ring burners with pans on. It’s like working next to a radiator, in a radiated kitchen."
Sue, a nurse from Blandford, just finished a night shift and has no idea how she'll get to sleep.
She says there is no air conditioning in her hospital building and they are handing out drinks by each bed to keep everyone hydrated.
Andrew, a head teacher of a primary school in Tottenham, says it's hard balancing the risk of having children in schools and the risks of sending them home.
He adds that buildings at the school "aren’t designed to handle this weather" and they don't have much shade in playgrounds "to keep people safe".
Matt McGrath
Environment correspondent
There's been a rare summer warning from Britain's electricity grid operator as the heatwave puts more pressure on our power system.
The National Energy System Operator (NESO) has put out what's called an "Electricity Margin Notice" for this evening, which means it is asking those generating electricity to make extra capacity available.
Such notices are normally seen in the winter, when our demand for electricity ramps up as we warm our homes on those cold, dark evenings.
Seeing one during a heatwave is unusual.
The pressure comes from both the supply and demand sides during the hot weather.
The warm, still conditions tend to suppress wind output, while high temperatures reduce the efficiency of solar panels and conventional power plants.
At the same time, demand can rise due to the extra electricity needed for air conditioning.
NESO stresses that this does not mean that blackouts are likely, or that there is too little power to keep the lights on, and it will review the situation through the day.
Image source, EPAThere's room for two under this umbrella in Margate
Image source, ReutersMore than just tennis fans on show at the Eastbourne Open today
It's a busy afternoon for this ice cream vendor at the Royal Norfolk Show
Image source, EPASee it, say it, sip it
Sarah Keith-Lucas
BBC Weather
Meteorologists here in the Met Office operations centre are very busy analysing huge quantities of weather data and observing closely how conditions are developing across the UK.
The forecasters here tell me the forecast is going as expected today, with temperatures rising quickly and humidity staying very high.
The next weather hazard they are looking into is the potential for some severe thunderstorms with large hail and the possibly onset of gale force gusts of wind for south-west England during Thursday night - a yellow warning has just been issued.
Image source, Met OfficeThe Met Office has issued a yellow warning for thunderstorms in south-west England on Thursday.
It will come into force between 18:00 and 23:59 BST, covering all of Cornwall and much of Devon.
Image source, House of CommonsOver in the House of Commons, the heatwave has just come up during this week's Prime Minister's Questions.
Responding to Liberal Democrat leader Ed Davey, Keir Starmer says the changing climate is "one of the most significant challenges of our time".
The prime minister says it used to be "common ground" that climate change was a "generational challenge", but that it is a "shame that in order to chase Reform votes" the Conservatives have "changed their minds".
"I will always maintain that we must be global leaders on climate change and we always will be," Starmer adds.
Hazel Shearing
Education correspondent
There's no set temperature at which schools are told they need to close in a heatwave - that decision is made on a case-by-case basis.
I asked Tim Warneford, whose consultancy helps schools obtain funding for their buildings, whether he expected quite so many to do so.
"I think it was a ticking time bomb," he replies, since England's ageing school estate was "not built to withstand the temperatures that we are now facing and are likely to face into the future".
"New buildings, we hope, will be designed sufficiently well that will reduce that risk, but the overwhelming number of our stock is going to require retrofit - and therefore that's going to require a certain amount of money," he adds.
A big problem is that many schools built in the 1950s and 1960s have "almost floor-to-ceiling windows" which trap in heat and are challenging to ventilate.
He adds that only schools built in the past decade or so are likely to have been designed with air conditioning units - and warns that some of those may not even be up and running because they're expensive to maintain.
Chris Fawkes
BBC Weather
Last night was the warmest on record for some UK sites, including at Brize Norton (Oxfordshire) where temperatures only fell to 22.3C - what is known as a tropical night.
From this warm and humid start, temperatures are rocketing upwards. Even before midday we've reached 31.6C at Thorney Island (Hants).
The June record of 35.6C is going to get smashed later this afternoon, with highs of 37 or 38C likely around 15:00 BST in southern England.
This would put us well inside the top ten hottest days ever recorded in the UK.
Global heating is to blame for such unprecedented heatwaves. You may remember that last month the May record was pulverised by well over 2C - it's clear that our climate has changed.
