A 14-year-old boy is believed to have drowned after entering the water of Tagg's Island in Hampton.
The London Ambulance Service was alerted on Monday, 18 July, after the teenager had difficulty in the Thames in Richmond.
Officers responded with paramedics and the London Fire Brigade.
The three emergency services and members of the public carried out an extensive search but were sadly unable to locate the boy.
The rescue operation was brought to an end at 18:40hrs. The rescue team said the search to recover his body will continue into this evening.
His next of kin are aware and are being supported by specialist officers.
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."©Standard Gazette, 2021. Unauthorized use and/or duplication of this material without express and written permission from this site’s publisher is strictly prohibited. Excerpts and links may be used, provided that full and clear credit is given to Standard Gazette with appropriate and specific direction to the original content.