Come Fly With Me star David Walliams is to swim around 200 miles - along the length of the River Thames.
The funnyman has previously swum across the English Channel and cycled the length of Britain for charity challenges. He announced that his next endurance test will see him following the course of the river from its source in the Cotswolds.
Walliams's challenge is to be staged for Sport Relief and will begin in September.
The star, who married model Lara Stone last year, revealed the mammoth journey as he spoke to Chris Evans on Radio 2. The river is around 215 miles long although it is not possible to swim the entire length of the route.
He joked: "You've got to walk the first (part) or run or jog - that's probably what I'll do, I'll jog. I'll jog when the cameras are on me and when they're off me I'll start walking."
Environmental campaigner Lewis Pugh completed the trip in 2006, but his start began with a 19-mile run because the river was barely a trickle. He completed the journey in 21 days.
Walliams will also take part in a challenge for next month's Comic Relief activities, which will see him hosting TV panel shows for 24 hours.
He said: "I'm going to be presenting a panel show for 24 hours. Basically all the current panel shows, Have I Got News For You, (Never Mind The) Buzzcocks, 8 Out Of 10 Cats - I'll be presenting all of them for a 24-hour period so I don't know how I'm going to be after about 12 hours - probably asleep.
"That's going to be streamed live and then highlights are going to be on the BBC on the night."
Walliams will team up with Evans on the judging panel for a children's writing competition, 500 Words, which is taking place at the Hay Festival of Literature and Arts. Children aged 13 or under are being encouraged to take part in the contest - launched on Evans's Radio 2 show this week - which is part of the BBC Year Of Books.