Little Britain star David Walliams will swim over 140 miles of the River Thames to raise money for Sport Relief in the UK.
The BT Sport Relief Challenge: Walliams vs The Thames begins on 5 September in Lechlade in Gloucestershire and it will take the actor over eight days to reach Big Ben in London.
Of his latest feat, he 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."
Walliams' swim is being filmed for a documentary which will air next year.
His previous adventures for charity include swimming the Straits of Gibraltar and cycling from John O'Groats to Land's End.
