Police divers are searching for three people who all went missing while swimming in the Thames, in three different stretches of the river.
The Metropolitan Police said a swimmer went missing at Shadwell Basin on Tuesday evening, a second at Waterloo Bridge and a third near Kingston High Street.
A spokesperson for the force, said: “Police were called... at 18:06 to Garnet Street (Shadwell Basin) to a report of a man seen to enter the water.
“The man, believed to be aged 22 years, was swimming with friends and has not resurfaced.”
Soaring temperatures in the capital on Tuesday meant thousands of people headed to the river to cool down.
Temperatures across England exceeded 30C (86F) on Tuesday, with forecasters predicting even hotter temperatures on Wednesday.
“Quite a lot of places are back up to 23 or 24 degrees already (at 5am),” Met Office meteorologist Emma Smith said.
“It’s normally 13 or 14 degrees at this time of year, so that’s 10 degrees above average.”
It will get to 35 degrees on Wednesday, with a small chance it will get to 36C,” she added.
The highest overnight average temperature ever seen in the UK was 23.3C (73.94F) in July 1948.
Smith said there is a possibility this will be beaten on Thursday night into Friday.
Temperatures in London are expected to reach 38C (100.4F) on Thursday, which would pass the current record for a day in July – 36.7C (98.1F) – recorded at Heathrow Airport in 2015.
The Met Office said there is a 40% chance the UK temperature record of 38.5C (101.3F), which was recorded in Faversham, Kent, in August 2003, will be exceeded.