A 14-year-old boy is believed to have drowned after getting into difficulty in the River Thames in Richmond.
A rescue operation was launched after the child was seen entering the water at Tagg’s Island in Hampton, west London, at around 4.45pm.
The Metropolitan Police has said the search was unsuccessful and the rescue operation was brought to an end after two hours.
Efforts to recover his body will continue into Monday evening, a spokesman for the force said.
Superintendent Richard Smith 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.”
The teenager is the latest in a number of fatalities during the heatwave in the UK, and police forces across the UK have urged people to avoid swimming in open water.
Supt Smith added: “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.”
It came after the family of 13-year-old Robert Hattersley said they were “absolutely devastated” after he died when he got into trouble in the River Tyne in Northumberland on Sunday.
Emergency services also confirmed the deaths of a 16-year-old boy in Salford Quays, Greater Manchester, a 16-year-old boy in Bray Lake near Maidenhead, Berkshire, and a 50-year-old man in a reservoir near Leeds in similar circumstances.
Detective Inspector Phil Hughes, of Leeds CID, said: “This incident (in Leeds) serves as a timely reminder about the dangers of swimming in open water.”
Northumbria Police said Robert’s death in Ovingham “does act as a poignant and timely reminder as to the potential dangers presented by water”.
The Crawcrook teenager’s family said: “It is impossible to put into words the heartbreak we are feeling. Robert was so kind and loving. We are absolutely devastated by what has happened. He brought a smile to so many people’s faces and he will be missed by absolutely everyone who knew and loved him.
“We’d like to thank all the emergency services who worked so hard to try to find Robert, as well as everyone who has been in touch passing on their messages of condolence.”
Police said Robert’s body was found after an extensive search, which started shortly before 4.15pm on Sunday.
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.”
