The record for the UK’s hottest June day has been broken for the third time on Wednesday, with provisional temperatures of 36C recorded at both Wisley, Surrey, and Gosport in Hampshire, the Met Office said.
Earlier a high of 35.8C in Wiggonholt, West Sussex surpassed the 35.7C record breaking temperature from this afternoon in Charlwood, Surrey.
During the peak of the heatwave on Wednesday and Thursday, the temperature could come close to the UK’s all-time high of 40.3C, which was measured in Lincolnshire in July 2022.
The latest heatwave is also expected to surpass the record for June of 35.6C set in Hampshire in 1976.
The Met Office has extended the rare red warning for extreme heat in place for Wednesday and Thursday down to the south coast of England, including Brighton, Bognor Regis, Portsmouth, Southampton, Bournemouth and Dorchester.
The weather service also said the red warning for the exceptional spell of hot and humid weather would now be in place until the end of Thursday.
Over 850 schools in England and Wales have told parents they will be fully closed or finishing early, with pupils at some schools have been told they can wear PE kit rather than full school uniform.
Great Northern, Southern, Avanti West Coast and Thameslink services will be disrupted until Friday, with Eurostar and Gatwick Express operators warning of delays. Chiltern Railways has axed more than half its services until Friday "to ensure the safe operation of the railway".
The Army has also cancelled ceremonial operations, including the Changing of the Guard at Buckingham Palace, in London and Windsor to "protect its soldiers' wellbeing."
The Red Alert has been extended to cover much of the south coast
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Avanti West Coast was the operator with the most disruption on Wednesday morning, with four out of five service either cancelled or more than 30 minutes late.
Chiltern Railways said it is running fewer than half its usual services until Friday "to ensure the safe operation of the railway".
This affects London Marylebone services to and from Birmingham, Oxford and Aylesbury via Amersham.
Great Western Railway issued a plea for its services to be used for "essential travel only" on Wednesday and Thursday.
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Much of London is sweltering in the heat
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A GWR spokesman said speed restrictions mean it will be forced to run fewer services than usual and journey times will be extended.
It is also running fewer trains than normal between London Paddington and Reading because Network Rail is not moving some points, pieces of metal that enable trains to change tracks, on the route to cut the risk of failures, which are more likely in high temperatures.
LNER said "we strongly advise that you do not travel" on Wednesday or Thursday.
An AA spokesperson said the breakdown callouts it is attending are "considerably more challenging than usual" because of high temperatures.
People have been forced to use umbrellas and shopping bags to shield themselves from the sun
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Britain’s electricity grid operator has issued an industry warning that the power supply could be squeezed on Wednesday evening as extreme heat puts pressure on the system.
It marks a rare summer call out for more power from the National Energy System Operator (Neso) as the country braces for record-breaking temperatures.
Neso, the body which is tasked with making sure the supply of electricity matches demand for it, said it had issued a so-called electricity margin notice.
This sends a message to the electricity market to ask for more capacity to be made available, allowing for production to increase if necessary.
Schools have been closed due to the extreme heat
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In Bristol, more than 60 schools are either closed or partially closed, with a further 30 closed or partially closed in South Gloucestershire.
There are also around 180 schools closed in Somerset, with at least 25 closed in Wiltshire.