Emergency service officials have warned against the dangers of taking a dip to escape soaring temperatures, as a number of people have died.
The mercury hit 38.1C in Santon Downham, Suffolk, making it the hottest day of the year and the third hottest day on record, after 38.7C in Cambridge in 2019 and 38.5C in Faversham, Kent, in August 2003.
The current UK temperature record looks almost certain to fall on Tuesday, when temperatures could reach 41C in Yorkshire and Lincolnshire, and hit 40C in London.
It comes as a 14-year-old boy is missing and believed to have drowned on Monday afternoon after getting into difficulty in the Thames in Richmond, west London.
The teenager was seen entering the water at Tagg’s Island in Hampton and, after a search took place, he was presumed dead, with officers calling the incident a “tragedy”.
Superintendent Richard Smith, from the South West Command Unit, said: “Despite the very best efforts of all involved, we must now sadly conclude that this young boy has died.
“His death is a tragedy and I cannot begin to imagine what his family will be going through. All our thoughts are with them.
“I know that on days like today when temperatures are at a record high, it might look appealing to jump in and cool off in rivers, reservoirs, lakes or other open water.
“Please don’t. The dangers are real and this evening in Richmond we have seen the terrible consequences of what happens when it goes wrong.
“To young people in particular, I would urge you to be the person in your group of friends who says no and reminds others about the dangers. Your intervention could save a life and save another family from experiencing such an awful loss.”
In Northumberland, the family of a teenager who died in the River Tyne after getting into difficulty have said they are "absolutely devastated".
Robert Hattersley, 13, of Crawcrook, got into trouble in the water near Ovingham, shortly before 4.15pm on Sunday 17 July.
Northumbria Police were joined by members of Tyne and Wear Fire and Rescue Service, Northumberland Fire and Rescue Service and HM Coastguard and Mountain Rescue for a search that continued into the night.
Police confirmed on Monday morning that a body had been found in the water.
Want a quick and expert briefing on the biggest news stories? Listen to our latest podcasts to find out What You Need To Know
It is also understood that a 16-year-old boy has drowned after getting into difficulty in a Maidenhead lake.
Thames Valley Police were called to Bray Lake at 11:45am where a widespread search was carried out.
Royal Berkshire Fire & Rescue Service, Buckinghamshire Fire & Rescue Service and the National Police Air Service (NPAS) also attended.
Upon arrival, crews used three rescue boats, a rescue sled, throw lines and water rescue poles. They were on the scene for around two hours and 30 minutes.
The body of a boy was found just after 1.30pm and he was pronounced dead.
Newly appointed Health Secretary Steve Barclay warned of “significant dangers” for people tempted to swim in a river to cool off amid the soaring temperatures.
Asked whether the public should be outside and visiting beaches, Mr Barclay told reporters people should use “common sense” and follow the advice of public health experts.
That meant “hydration, covering up, being in the shade, avoiding the times of the day when heat is at its peak”.
He added: “There is a particular message, particularly for teenagers, children, some of those who may be tempted to go for a swim – there’s significant dangers of that, quite often when people go swimming in rivers when we have very hot weather.
“So it’s following common sense steps and keeping an eye on neighbours and those who are vulnerable, and following the guidance that’s been put out by the relevant bodies.”