A yellow weather warning has been issued for thunderstorms in parts of south-west England and much of Wales, the Met Office said.
The warning is in place between 3pm and midnight, and the forecaster has warned of heavy rain, hail and spray which could affect drivers.
The warning also means there will likely be some damage to buildings from lightning strikes and delays to public transport services.
The Met Office said some localised flooding is possible as are short-term power cuts.
It comes after Tuesday was the hottest day in May on record for both England and Wales, according to the Met Office, with Kew Gardens provisionally reaching 35.1C and Cardiff Bute Park reaching 32.9C.
Meanwhile, fresh warnings have been issued over the dangers of open water after several teenagers died after getting into trouble in recent days.While many people enjoyed the sunny bank holiday weekend, older people and babies can be particularly vulnerable.
There is also a risk of dehydration and heat exhaustion, which is caused by the body losing too much water and salt, and can lead to heatstroke, a life-threatening illness.
NHS England said there were 20,092 visits to its heatstroke advice page on bank holiday Monday, compared to just 488 the previous Monday. Over the whole weekend, there were 36,724 hits.
Elsewhere, people looking for tips on how to deal with sunburn led to 5,342 visits to NHS webpages, with a total of 10,314 over the whole weekend.Parents also sought advice on how to keep their babies safe, with visits to NHS web pages for baby first aid and sun safety tips surging by almost 3,500% on Monday, with 4,728 hits.Temperatures are expected to cool slightly in parts of the UK in the coming days.However, health officials have confirmed that heat health alerts would be extended for 24 hours.
The UK Health Security Agency said amber alerts for the South West, South East, London, East and West Midlands and the East of England will remain in place until 5pm on Thursday, with yellow heat-health alerts for the North West and North East.
Duncan Burton, chief nursing officer for England, said: “While many enjoyed the rare treat of a sunny bank holiday, the soaring temperatures have been of real concern for many new parents, with a 3,500% increase in the number seeking NHS advice for how to keep their babies safe in the sun over the weekend.
“Temperatures are now starting to ease off, but there are still health alerts in place for several parts of the country until 5pm Thursday, so it’s vital that people stay on their guard if they’re looking after those who are vulnerable to the heat.“Babies, children, older people and those with long-term conditions like diabetes or heart problems are at higher risk of heat exhaustion or heatstroke, so we’d urge people to take care and to check on their loved ones during these high temperatures.“
The NHS website has a range of useful information for young children of different age groups to help people parents their loved ones safe and avoid them becoming dehydrated.”
In recent days people have died after struggling in open water in South Yorkshire, West Yorkshire, Devon and Cornwall, Warwickshire, Lincolnshire and Lancashire.On Wednesday, Cheshire Police said the body of a 17-year-old boy was found after he went missing in Pickmere Lake at Marston, Northwich.Officers carried out extensive searches of the area to locate the 17-year-old, supported by the police helicopter, Cheshire Fire and Rescue Service, and specialist Underwater Search Teams.
Chief inspector Jamie Lewis said: “We understand that with the warm weather many people could be tempted to take a dip in open water to cool off, but residents must be aware of the potential deadly consequences.“We don’t want to discourage people from enjoying themselves near water, but there can be hidden currents and unseen dangers such as mud banks or items under the water that a swimmer can become caught on.”
A body has also been found in the search for a teenage boy who went missing after being last seen swimming at a boating lake on the Hampshire/Surrey border.
Hampshire Constabulary said a multi-agency search was launched on Tuesday afternoon following the disappearance at Hawley Lake near Blackwater.Declan Sawyer, 15, died after getting into trouble at Swanholme Lakes, Lincoln, on Sunday.
A 13-year-old boy, named locally as Reco Puttock, died after getting into difficulty at Leadbeater Dam, near Halifax, West Yorkshire, on Monday and the body of a teenage girl was recovered from the water at Kingsbury Water Park, Warwickshire.
The same day, a man in his 60s died of cardiac arrest after entering the sea at Tregirls Beach, Padstow, to help two family members who were in difficulty, Devon and Cornwall Police said.The body of a teenager was recovered from the water at Rother Valley Country Park, Rotherham, in the early hours of Tuesday morning, South Yorkshire Police said.
And on Tuesday evening, Lancashire Police said a body had been recovered from the River Ribble after a 12-year-old boy got into difficulty while swimming with friends at Ribchester.
The RNLI warned of the “very real risk” of swimming in open water during the heatwave and said that while temperatures have hit record highs for May, water temperatures remained low and this could lead to cold water shock.
