Amber alerts issued by the UK Health Security Agency (UKHSA) for the West Midlands and South West, and yellow alerts for the East Midlands, the North West, the South East, the East of England and London, will remain in place until 9pm Wednesday.
The UKHSA said significant impacts are likely across health and social care services and warned of a potential increase in water-related incidents across England, including risks from risks from cold water shock and drowning.
It comes as two teenagers have died in open water in separate incidents during the latest heatwave.
Police were called to Dovestone Reservoir, Oldham, at around 7pm on Friday.
They found a man, 18, who was unresponsive in the water. Despite the efforts of medical staff, he was pronounced dead at the scene, Greater Manchester Police (GMP) said in a statement.
Detective Inspector Steven Horton, from GMP’s Oldham district, said: “We have tragically recovered the body of an 18-year-old man from the water, and are conducting extensive enquiries to establish the full circumstances.
“Our thoughts remain with his family and friends during this incredibly difficult time.
“I would like to take this opportunity to remind the public of the dangers of open water. We strongly encourage people to stay safe and refrain from entering large bodies of water, including reservoirs, rivers, canals, or ponds.”
Derbyshire Constabulary announced on Friday that the body of an 18-year-old man had been recovered from the water near the weir at the Darley Abbey Mills complex after two teenagers got into difficulty.
The county’s fire and rescue service said it responded to a call at 12.46pm. The operation included water rescue units and specialist trained police divers, with police confirming the recovery of the body at 7.45pm.
The two incidents are just the latest in a spate of open water deaths this summer during successive heatwaves.
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During the May heatwave, at least 15 people, mostly children, drowned while swimming in open water.
Forecasters have said areas of England and Wales will continue to exceed 30C throughout the weekend, and into next week, as it was announced that 2026 had become the first year to record temperatures of 35C on six separate days.
This year has also become the first to see temperatures of 35C or higher in three calendar months – and broke the record for most temperatures of 34C or higher, with nine so far this summer.
The London Fire Brigade (LFB) has urged people not to use disposable barbecues over the weekend over fears of the dry heat leading to fires.
Water firms have also announced hosepipe bans for the east of England, Cambridge, Hampshire, the Isle of Wight and areas of Kent.
