Comedian David Walliams has been advised not to swim through the London section of the River Thames due to the amount of sewage dumped in it recently.
He is swimming along the river for the Big Splash Challenge for Sport Relief and is due to arrive in the capital in the next few days.
However, Thames Water said about 500,000 cubic metres of raw sewage had entered the river since Monday.
Sport Relief said Walliams still intended to swim the London section.
The Little Britain star Walliams has fallen behind schedule in his charity eight-day swim along the Thames.
He began in Gloucestershire on 5 September but suffered a high temperature and struggled to keep up his pace.
Despite this, the 40-year-old managed to rescue a dog as he swam between Maidenhead and Windsor on Saturday.
Vinny, a one-year-old Labrador, was struggling to get out of the water due to a bad hip.
Walliams said: "I heard a splash and looked over to be met with the face of a sweet Labrador.
"I thought he was fine, but he started to really struggle when he was getting out so I swam over and helped his owner get him out."
To add to the comedian's problems, heavy rain in the London area this week has seen a large increase in the amount of effluent being dumped in the river.
About 39 million tonnes of sewage is discharged into the river each year, due to a lack of capacity in the capital's Victorian sewerage network.
Tidal conditions mean it could take as long as a month for all of the waste to reach the sea.
Richard Aylard, of Thames Water, said: "We've been in touch with David Walliams' team and he'll have to make his own decisions.
"We're not public health experts but I wouldn't recommend swimming in it.
"That said, David is doing a hugely admirable thing and we wish him all the best."
A Sport Relief spokesman said: "David underwent extensive physical assessments to prepare him for this challenge, and received a number of inoculations to protect him from disease carried in the Thames.
"A team of medics are with him throughout the swim and are constantly monitoring his health.
"David's safety has been paramount throughout the course of his swim."