A body has been found after three people went missing in different parts of the River Thames on Tuesday.
Police believe it is a 23-year-old man who vanished while swimming with friends near Shadwell Basin in Wapping.
The Met said next-of-kin had been told and searches were continuing for two men who went missing near Waterloo bridge and in Kingston upon Thames.
With Britain braced for record-breaking heat, The RNLI has warned against cooling off in lakes and rivers.
Police said the man’s death was not being treated as suspicious and that a file would be prepared for the coroner.
More swimmers were seen entering the water at Shadwell while police officers and a forensic tent remained at the scene.
BBC London’s Greg Mckenzie said when asked why they would risk their safety, a group of teenagers replied: “We swim here every day.”
As temperatures in London reached 32C, thousands of people headed for the river.
But lakes and rivers remain cold and can “literally take your breath away”, the RNLI warned in a tweet.
London’s new swimming lake in Beckenham Place Park has closed just five days after it opened so new safety measures can be put in place.
A spokesperson for Lewisham Council which is responsible for the park, said: “In order to manage the numbers of swimmers and lake users safely we are introducing some temporary fencing around the lake perimeter to restrict the amount of people in the water at any one time.”
Parliament Hill Lido in Hampstead Heath, north London, has also closed “until further notice due to crowd numbers”.
Hampstead Heath said it was unlikely to re-open “at this stage with the numbers we have”.