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.
The Met echoed this warning, with Insp Stuart Simpson, from the Met’s marine policing unit, saying: “Whilst at times, the Thames may look appealing, especially in this hot weather, it remains very dangerous all year round.
“On initial entry the water can seem warm on the surface, but further in it can be freezing cold and there are often very strong undercurrents.
“The initial shock of the cold water is often what leads to people going subsurface and subsequently drowning.”
He added that the Met’s thoughts go out to the friends and family of the discovered man “at this extremely upsetting time”.
Police said the his death was not being treated as suspicious and 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 on Tuesday, thousands of people headed for the river.
Forecasters say temperatures in some parts of the UK could reach 36C (97F) later on Wednesday, while Thursday could be the UK’s hottest day on record as temperatures could reach a record-breaking 39C (102.2F) in London.
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”.