Police have named an 11-year-old boy who went missing in the River Don in South Yorkshire as Mackenzie Swift as searches for him continue.
Mackenzie was spotted going into the water off Ferry Boat Lane in Mexborough at about 20:00 BST on Saturday and had not been seen since.
South Yorkshire Police said his family were "living every parent's worst nightmare", with searches including an underwater team and Mountain Rescue volunteers.
"Our family liaison officers have sadly had the heartbreaking conversation with Mackenzie's family that if he had not exited the river on Saturday, he could not have survived in the water for this length of time," a force spokesperson said.
"Sadly, none of our enquiries so far have found any indication that he got out of the water."
Mackenzie's family were being supported by officers, they added, with police urging members of the public to stay away from the area.
Officers would remain at the site overnight, police said, with further searches resuming at first light on Tuesday.
Chief Supt Pete Thorp said: "I understand members of the public are desperate to help, but I must ask that you leave this work to the professional search teams.
"Please do not put yourselves or others in danger by searching in and around water, and if asked to leave by officers, please understand this is for your own safety and to ensure our efforts focus solely on finding Mackenzie."
He added: "The thoughts of all those involved in the extensive search operation remain with Mackenzie and his family."
There have been at least 15 water-related deaths during the recent heatwave in the UK.
A 13-year-old girl died on Sunday after going missing in a river in North Yorkshire.
The teenager was found in the River Wharfe, near Burnsall, in the Yorkshire Dales, and later died in hospital.
A 15-year-old girl from Cheshire died in hospital on Saturday after getting into difficulties in the sea off the coast of Merseyside on Bank Holiday Monday.
A woman in her 60s died on Saturday after she and a man got into difficulty after entering the water in Thornton Cleveleys to "rescue their dog".
Lancashire Police said the man, also in his 60s, remains in a critical condition in hospital.
Meanwhile, a 19-year-old man died after being rescued from a lake in Nottinghamshire, and a body was found in the search for a man who got into difficulty in the water in the Norfolk Broads.
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