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 and panel show host will set off from Lechlade in Gloucestershire and will swim 140 miles over eight days to reach Big Ben in London.
Walliams' epic swim 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.
As well as carefully navigating the busy river traffic, Walliams will have to deal with murky water that harbours a cocktail of bacteria such as E.coli, salmonella and hepatitis. He will also have to avoid the 39 million cubic metres of raw sewage that finds its way into the Thames every year after heavy rainfall.
The comedian said: "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.
"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.
"And aside from anything else, you can't see anything. There's zero visibility in the water - I won't even be able to see the end of my arm."
The BT Sport Relief Challenge: Walliams vs The Thames, which begins on September 5, will be filmed for a documentary to be broadcast in the build up to the Sport Relief Weekend in March next year.
Walliams is no stranger to getting wet for a good cause - in 2006 he swam the Channel, raising £1 million in aid of Sports Relief. Walliams is also asking for people to sponsor him at www.sportrelief.com/walliams.