Thursday could be the UK’s hottest day on record, forecasters have said.
Temperatures in London could reach a record-breaking 39C (102.2F) on Thursday, beating the previous all-time high of 38.5C recorded at Faversham in Kent in 2003.
The Met Office said there is a 60% chance that the record will be broken.
Thunderstorms swept across much of the UK on Tuesday night, ahead of the possible record-breaking heat, with 48,000 lightning strikes reported.
Forecasters say temperatures in some parts could reach 36C (97F) later on Wednesday. Northern Ireland and western Scotland are expected to be the coolest areas with temperatures in the low 20s.
The Met Office has issued a yellow weather warning about thunderstorms which are expected to hit most of England and southern Scotland from 4pm on Thursday and last into the early hours of Friday morning.
The NHS has tweeted advice for dealing with the unusually hot temperatures and said: “Try to avoid spending extended periods in the sun this week. Also, be aware that vulnerable people are at increased risk of health issues.”
The London Ambulance service added: “If catching a train or the Tube in this hot weather make sure you carry a bottle of water and step off the train at the next station if you feel unwell.”
Network Rail has announced that speed restrictions are to be introduced on Thursday because of the heat.
In a statement, Network Rail said: “We may have to introduce speed restrictions during the hottest part of the day at vulnerable locations as slower trains exert lower forces on the track and reduce the likelihood of buckling.
“Passengers should check before they travel, by visiting their train operator’s website or National Rail Enquiries.”
Rail operator Southeastern has said it will run a “significantly reduced service” on Thursday due to the speed restrictions.
Extreme weather action teams (EWATs) have been “activated” to keep passengers safe and trains running, Network Rail added.
Elsewhere, passengers on a Eurostar train from London to Brussels were stranded in 38C weather on Wednesday morning, after their train broke down due to an overhead power supply problem.
The train was evacuated and passengers were given bottles of water while they waited for another train to collect them.
Houses in Cheshire, Warwickshire and north Wales caught fire after being hit by lightning, while a taxi driver had to be rescued from flood water in Newbridge, Caerphilly county.
Megan Zahra, from Wrexham, said a lightning strike sounded “like a gunshot” when it set her bedroom roof on fire.
She woke up but thought it was a noise from outside and did not know the roof was on fire until her stepfather arrived to get her out.
Ms Zahra, 23, told BBC Wales: “I heard this big… it was like a gunshot or an explosion. I thought it was just outside here so I checked the window and didn’t see anything so I got back into bed.
“About 15 minutes later my stepdad was outside going, you need to get out, it’s on fire.”
Police divers were called out on Tuesday evening to three different stretches of the Thames in Greater London, following reports of people getting into difficulty in the water.
The Metropolitan Police said on Wednesday that the body of a 23-year-old man had been recovered from the river near Wapping, east London.
A spokesman said: “The death is not being treated as suspicious. Searches continue to locate two other males currently missing in the Thames.”
Two lightning spotters in Lancashire had to be rescued from a beach by police when their car got stuck amid a “spectacular” overnight thunderstorm.
Lancashire Road Police said the pair were parked on sand off Carr Lane in Middleton, near Morecambe, when the car became embedded.
Officers helped release them before the tide came in and have urged people not to try and drive on to beaches.
Lightning damage to a signalling system at Wakefield Westgate station in west Yorkshire led to fewer trains running on Wednesday morning, train operator Northern said.
Damaged signalling equipment has also affected trains in the Highlands and cross-border services between Scotland and England.
The French city of Bordeaux hit its highest temperature since records began on Tuesday – with Meteo France registering 41.2C (106.1F).
A World Meteorological Organization spokeswoman said the heatwaves bore “the hallmark of climate change”.