More than 300 schools across England and Wales are planning to be either partially or fully closed on Tuesday and later this week because of the weather.
Others are also closing early on Wednesday and Thursday to protect children from the heat, with some pupils being told they can wear PE kit rather than full school uniforms for those days.
The Department for Education does not itself “normally advise schools to close” and instead issues guidance on how best to manage high temperatures.
There will also be travel disruption for those travelling by car, rail and air, the Met Office warned.
On the National Rail network, passengers have been urged against undertaking non-essential travel and to make sure they are carrying water.
GTR – the UK’s largest train operator – said it will suspend the Gatwick Express from early afternoon on Wednesday and Thursday. The airport will still have services via Thameslink and Southern trains.
The operator will also implement safety speed restrictions across the next two days, with fewer trains on Thameslink, Great Northern and Southern lines.
GTR passengers can get refunds for tickets they have bought for Tuesday and the next couple of days if they will no longer be travelling.
Avanti West Coast also said it would run fewer train services between Tuesday and Thursday, while Chiltern Railways has reduced its services by more than half over those three days.
National Rail explained that heat can cause a number of issues including overhead lines expanding and rails buckling.
The RAC said it is anticipating breakdown assistance will “increase significantly as temperatures peak”, adding that it was already dealing with 10% more breakdowns than usual for this time of year.
Less severe amber weather warnings are in place until Thursday for parts of south and central England and parts of Wales.
Tuesday’s hot and humid weather was prefaced by thunderstorms in the early hours, with some 29,000 lightning strikes recorded in the south, the Met Office said.
The storms caused flash flooding and travel disruption across parts of England, with the London Fire Brigade reporting that it responded to 400 calls overnight.
Emergency services said two house fires in London and one in Bristol were believed to have been caused by lightning strikes.
There were also power outages in south-west England, with hundreds of homes briefly without electricity on Monday evening, according to the National Grid.