Little Britain star David Walliams will brave raw sewage and deadly diseases to raise money for Sport Relief by swimming the entire length of the Thames.
The comedian will set off from Lechlade in Gloucestershire and will swim 140 miles over eight days to reach Big Ben in London.
David's epic journey will also see him brave bitingly cold river water, which can cause cramp and involuntary breathing spasms known as a "gasp reflex".
He will battle powerful and unpredictable currents and undertows and will burn the equivalent of 4,400 calories every day.
"Most people in the UK know the Thames, and most people think the idea of swimming in it is horrifying. And it is.
"It'll be relentless. It's a huge distance so I'll need all the support I can get," he confessed.
"It's really not a safe environment to swim in - people really mustn't try this, I have an incredible team of experts to help keep me safe.
If there's a lot of rainfall there'll be a lot of sewage present so I've had to have loads of jabs."