It is the third day of a red warning for extreme heat in the South.
On Friday the warning is in place for parts of Hampshire, Berkshire and Oxfordshire with an amber alert for a wider area until midnight.
The search for a 15-year-old boy missing after going swimming in a lake is continuing at Testwood Lakes.
A new record June temperature of 36.1C (87F) that was set in Gosport, Hampshire on Wednesday was eclipsed by Yeovilton, Somerset on Thursday.
The NHS in Hampshire alongside hospitals in Southampton and Portsmouth and South Central Ambulance Service declare critical incidents.
Several rail operators have extended their reduced service due to the extreme heat into Friday.
About 200 schools across the region have announced closures or reduced hours due to the heat.
There were thunderstorms with dramatic lightning in Dorset on Thursday night.
Follow this advice to keep yourself and your home as cool as possible.
Image source, PA MediaPastel parasols are out in force as visitors to Windsor shelter from the sun.
Due to the extreme heat there are no ceremonial guard changes at the castle, external on Wednesday or Thursday they have also been changed or cancelled in London.
The Household Division, a group of seven army regiments said: "The wellbeing of our soldiers, horses and the public will always come first."
Water Rescue team, Wessex Rescue said its team of volunteers had been put on standby to support the emergency services in Dorset and the surrounding counties as the red warning was now in place.
The charity said as well as its water rescue team it was also able to offer support including drones, communications, incident management, and volunteers to help the community if needed.
Image source, Hampshire County CouncilGritters are ready to spread crushed stone onto roads across Hampshire where the ongoing extreme heat could lead to road surfaces softening and becoming uneven.
Hampshire County Council, external said the prolonged hot weather means road surface temperatures could become extremely high – reaching the high 50sC to low 60sC.
It said roads were being closely monitored with its fleet of gritters ready and waiting.
The council also urged drivers to check travel conditions before leaving and where it is safe to do so during a journey, take plenty of drinking water for all passengers and to consider travelling earlier or later in the day to avoid the hottest periods.
Image source, Met OfficeThe Met Office has extended the rare red warning, external for extreme heat in place for Wednesday and Thursday down to the south coast including Portsmouth, Southampton, Bournemouth and Dorchester along with Brighton and Bognor Regis.
The weather service also said the red warning for the exceptional spell of hot and humid weather would also now be in place until the end of Thursday.
While dozens of schools across the south of England have fully or partially closed - two in Dorset are keeping lessons going with shade, ice lollies and looser uniform rules in place.
At the Prince of Wales School in Dorchester, classes have spilled out into shaded areas of the playground, with pupils swapping the indoors for cooler outdoor spaces wherever possible.
Head teacher Gary Spracklen said the priority was to adapt rather than close and believed it was "important to always be open".
At Wyke Regis Primary in Weymouth, a sea breeze is helping and staff and pupils are taking a slower pace through the school day.
Image source, Cowes Harbour CommissionWith temperatures rising people are being urged not to be tempted to jump into the water at Cowes Harbour.
Cowes Harbour Commission , externalsaid swimming is not allowed in the harbour area and the water is still cold only averaging around 12C and jumping in could trigger cold water shock.
The commission said "If you're planning a swim, please use one of the local beaches around Cowes and East Cowes, wear an appropriate wetsuit, check conditions before entering the water, and consider using a flotation device."
Lots of train operators are warning people not to travel today unless it's absolutely necessary.
Our correspondent Nick Johnson was at Reading Station earlier where he said the station was "much quieter" than usual and it seemed there were fewer people travelling.
Great Western Railway the main operator out of the station has urged people to only travel if it was absolutely essential with fewer trains and those that are running having to run at slower speeds.
Reporting during BBC Breakfast, Johnson said rail companies had also warned services may have to be "cancelled at short notice especially during the hottest parts of the day."
A rare red weather warning has just come into effect and will remain in place until 21:00 BST on Thursday.
It covers parts of Dorset, Hampshire, Berkshire and Oxfordshire, as well as the Midlands and south Wales, with people in those areas warned there is a risk of serious illness or danger to life.
Image source, PA MediaRail operators have urged people to avoid travelling by train within the red weather warning zone on Wednesday and Thursday and are warning those that do to prepare for a disrupted journey.
If you have a ticket for travel on Wednesday 24 June or Thursday 25 June, you can use it on Friday 26 June instead without changing your ticket.
Image source, PA MediaDozens of schools across the BBC South region have planned for at least partial closures over the next two days due to the high temperatures.
Schools usually contact parents in their own ways, often through apps or messaging services.
Hampshire County Council, external and West Berkshire Council, external publish lists of reported emergency closures.
Oxfordshire County Council, external also shares a list of schools that have closed suddenly, based on information provided by the schools themselves.
Children at some schools remaining open have been told they can wear PE kit rather than full school uniform.
Our analysis suggests more than 140 schools in the South were closed or partially closed on Tuesday because of the high temperatures.
Indy Almroth-Wright
BBC South
Image source, BBC Weather Watcher S PaceIt's Wednesday and the south is braced for extremely high temperatures with schools closing and people urged not to travel, as a rare red heat weather warning comes into force.
The Met Office red warning, external for extreme heat begins at 09:00 BST an amber warning is already in place.
The latest heatwave is also expected to surpass the record for June of 35.6C set in Mayflower Park, Southampton in 1976.
We’ll be bringing you the latest throughout the day.
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We're going to take a break, fan ourselves, watch the World Cup, and return in the morning.
The rare red warning for extreme heat lasts into Thursday, so take care of yourselves.
West Berkshire Council says it is "working closely" with schools as they "respond to the exceptionally hot weather".
The authority says its position is that schools "should remain open wherever it is safe and practical to do so", but recognises they are facing "challenging conditions" and are best placed to make decisions "based on local circumstances".
"We fully support schools in taking any necessary steps to protect the health, safety and wellbeing of pupils and staff.
"We remain in regular contact with schools and colleagues across public health and health and safety services, and we are grateful for the swift and responsible way school leaders have responded.
"Our message is clear: safety comes first, and we will continue to support schools in making the right decisions for their communities."
Image source, National TrustThe National Trust is set to open some of its Dorset properties - Max Gate, Hardy’s Cottage and Clouds Hill - during the mornings only on Wednesday and Thursday.
National Trust North and West Dorset, external says with temperatures expected to be exceptionally high, it wanted to ensure the safety and wellbeing of our staff, volunteers and visitors.
"Closing during the hottest part of the day will help reduce the risks associated with extreme heat and allow everyone to enjoy these special places more comfortably," it says.
In Dorset, emergency support (SWEP) has been activated to support people sleeping rough due to the extreme heat.
Dorset Council says it will stay in place until Thursday 25 June, external, when it will be reviewed.
The authority says: "We've asked our providers to make water and sunscreen available and encourage individuals to seek shelter during the height of the day."
Alex Meakin
Reading
Mabel Gregory is the manager of the St Saviour's drop in centre for the homeless in Reading.
It's one of the places that Reading Borough Council is directing the homeless community to as part of its severe weather protocol.
She said: "It's too hot, they don't really like it.
"You and I can go home and sit in our nice cool dining room. These guys, they have to find shade somewhere. But we'll make sure they're fed and washed."
Image source, GoogleSouthampton City Council says the “safety, health and wellbeing” of children, young people and staff is its priority during the heatwave, amid calls from the National Education Union to close schools.
Amanda Barnes-Andrews, the council's cabinet member for children and learning, says the authority is working with schools to assess the situation based on their individual circumstances and national guidance.
"Having completed risk assessments, some schools may need to decide to close," she says.
She adds that closures are not expected nationally, but schools can take steps such as adjusting timetables, relaxing uniform rules and ensuring access to water and cooler spaces.
Frank Long, group manager at Royal Berkshire Fire and Rescue Service, has been speaking about the risk of fires in the open, and wildfires.
"The ability for small fires to grow into very large problematic fires is a real cause of concern for us," he told the BBC earlier.
Berkshire fire manager discusses wildfires
Weather Watcher BrotherBart has taken a picture of his house thermometers sitting in his back garden in Cosham, Portsmouth, and they're showing some alarming temperatures!
Image source, BrotherBartMeanwhile Weather Watcher Yen Milne has attempted to fry an egg in their garden in Fareham.
If anyone else wants to try this at home do not expect us to egg you on.
Image source, Yen MilneWeather forecast on Tuesday, 23 June
Overnight, temperatures will remain high and conditions will stay warm and muggy under clear skies. Winds will be generally variable or light.
Wednesday will remaining exceptionally hot and humid - even hotter than the previous day for many.
Winds will remain variable or light and there may also be a few isolated patches of cloud around.