Three people pulled from the water died on Sunday as crowds flocked to beaches and waterways looking for relief from the heatwave and temperatures are set continue to soar.
Teenagers died in Greater Manchester and Oxfordshire, along with a man in his 50s in North Yorkshire after the emergency services rescued them from water.
Temperatures are set to sizzle again on Monday with a scorching 32C forecast and the hot weather is expected to remain until Thursday.
Beaches and parks are expected to remain crowded as workers either call in sick or book the day off, while many families also keep their children off school to avoid self-isolation ahead of the summer holidays.
A packed beach in Weymouth, England
Sunday was the hottest day of the year so far in both England and Wales, the Met Office said, with temperatures reaching 31.6C at Heathrow and 30.2C in Cardiff.
On Monday low cloud in the northeast will break to allow sunny spells, but it will be cooler along North Sea coasts, while hot sunshine is forecast for southern and western parts.
Greater Manchester Police said that despite a rescue operation at Salford Quays, a 19-year-old man had died.
The force tweeted: "Our thoughts are with this young man’s family and friends, and the people who witnessed the tragic events.
"We will provide further updates when we have them."
A warm start to Monday morning with strong #sunshine burning off any mist and rapidly lifting temperatures ☀️📈🌡️
Cooler and cloudier in the northeast and also the far northwest ☁️ pic.twitter.com/OxKGQGDTKz
— Met Office (@metoffice) July 18, 2021
North Yorkshire Police tweeted: "Sadly, the body of a man in his 50s was recovered from the River Ouse, close to the Water End Bridge in York this evening. Officers attended the scene, along with the fire and ambulance services, after being contacted by kayakers shortly before 5pm.
"While formal identification is yet to take place, we believe we know the identity of the deceased but are awaiting confirmation. His next of kin have been informed and our thoughts are with them at this very sad time."
Thames Valley Police (TVP) said a teenage girl had been brought from the water in Witney, near Oxford.
"As you might be aware, our officers attended Ducklington Lake, in Witney at 2.35pm today (18/7) after a fear for welfare report concerning a teenage girl in the water," TVP West Oxon tweeted.
"Officers attended with both fire and ambulance services, and the girl was brought from the water and taken to hospital. Tragically, the girl later died in hospital.
Pictures: Extreme weather around the world
"The girl’s family have been informed, and are being closely supported by specially trained officers. The death is being treated as unexplained and non-suspicious, and a file will be prepared for the coroner.
"We are very sorry to bring you this tragic news. Our thoughts are with the girl’s family and loved ones at this incredibly difficult time."
The deaths on Sunday come after Cumbria Police said the body of a teenage boy, who entered the water about 9pm on Saturday, had been found in the River Eden, near Stony Holme, at about11.45am on Sunday.
The heatwave also means some staff due to return to the office on Monday as restrictions are finally lifted are likely to enjoy the sunshine instead of returning to work.