David Walliams has begun the seventh day of his bid to swim the River Thames from its source to central London.
The comedian entered the water at 06:30 BST at Old Windsor Lock, in Berkshire, and aims to swim 21 miles to Teddington, in south-west London.
He is continuing his Big Splash Challenge for Sport Relief despite being warned not to swim in the river.
Thames Water said 500,000 cubic metres of raw sewage had entered the London section of the Thames this week.
Sport Relief said Walliams had received several inoculations to protect him from diseases carried in the river, and his health was constantly being monitored.
The 40-year-old Little Britain star has fallen behind schedule in his bid to swim the 140-mile route in eight days.
He began at the river's source in Gloucestershire on 5 September but suffered a bout of "Thames Tummy" which left him with a high temperature.
Walliams said he had recovered from the illness but was wary of the challenges ahead.
He said: "We have broken through the 100 mile barrier now which is great, but once I reach Teddington Lock the Thames becomes tidal and there's a whole new set of challenges.
"The currents and undertows are very dangerous so I'll need to be strong."