Temperatures are set to soar to highs of 33C on Thursday amid a continuing heatwave, prompting warnings of a “significant health risk”.
Public Health England (PHE) said it was “vitally important” that people look out for vulnerable friends, families and neighbours as the hot weather intensifies.
The alert came as a 17-year-old boy died after getting into difficulty while swimming in the River Aire in Leeds.
The Met Office said temperatures climbed to 31.9C in Wales – making Wednesday the hottest day of the year so far.
“There’s a good chance today we might well see something a little higher in the northwest,” said forecaster Simon Partridge. “It is going to be dry, fine and sunny.”
Temperatures are likely to peak on Thursday, with possible highs of 33C, soaring above Britain’s June average of 17.3C.
The Met Office’s heatwave alert is currently at level two, meaning social and healthcare services should prepare to take action to reduce harm.
PHE warned the elderly, children and people with heart conditions were most vulnerable to health problems during the heatwave.
Dr Thomas Waite said: “We know that when weather like this hits, many people will head outdoors and make the most of the sunshine – but for others, temperatures like these, over more than a day or two, can be really uncomfortable and pose a significant risk to health.
“This is because their bodies may struggle to adapt to working harder, as all our bodies do when the weather gets this hot, and they can become ill.
“It’s vitally important that we keep an eye on friends, family and neighbours who may be at risk.”
Charities have also warned that homeless people were at risk if exposed to prolonged sunshine and heat while sleeping rough.
“A lot of people sleeping rough don’t have the basic items needed to survive on the streets in hot temperatures,” said Debra Ives, director of operations at Evolve Housing and Support.
“As a result, we’re calling on the public to donate things like sun cream, water and sun hats, to help those at risk in the coming months. People can either offer them directly, or donate them to a shelter or charity.”
The heat has fuelled wildfires in northwest England and has caused travel disruption as train speed restrictions are enforced to prevent steel rails buckling.
Rising temperatures have also prompted police warnings about cooling off in open water after a man drowned swimming in a lake in Nutfield, Surrey, on Monday evening.
A search is ongoing for missing 13-year-old Ryan Evans, who got into difficulty in a lake in Stoke-on-Trent on Monday.
And an unnamed teenager disappeared while in the River Aire on Tuesday evening.
Fire and rescue crews trawled the river in Leeds at the spot where the boy was last seen for over 90 minutes but did not find anyone in distress.
The RNLI has urged those heading to the seaside to seek out beaches with a lifeguard.
Greater Manchester Police declared a “major incident” after a huge moorland fire spread towards homes, forcing people to evacuate.
The army was called in to help as firefighters battle flames on Saddleworth Moor for a third day.
Derbyshire Fire and Rescue Service reminded people that “a stray cigarette, a discarded glass bottle or a spark from a BBQ can all cause untold devastation”, and neighbouring Nottinghamshire Fire and Rescue said it dealt with a number of grass and rubbish fires.
In York, police smashed the window of a vehicle in a hospital car park to rescue two dogs from sweltering conditions inside.
Network Rail’s extreme weather action teams have been mobilised across the country to monitor “vulnerable locations”.
Services on the London Waterloo to New Malden line are being subjected to speed restrictions from 11am each day until Friday.
Network Rail said hot weather can cause the steel on rail lines to expand and in some cases buckle, causing travel disruption. Slower trains exert lower forces on the track, reducing the likelihood of buckling.
Mr Partridge said temperatures “will ease off slightly into the weekend, but not a great deal, with most places around the mid-20s as we get a bit of fresher air coming in”.
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