Comedian David Walliams is set to swim about 140 miles of the River Thames in just eight days for Sport Relief.
Starting on 5 September, the Little Britain star will swim from Lechlade in Gloucestershire to Big Ben in London.
The bid is part of the Big Splash campaign to get the nation swimming.
Walliams, 40, will pass through several major towns en route, including Oxford, Reading, Henley, Maidenhead, Windsor, Kingston and Richmond.
In 2006, Walliams completed a swim across the English Channel, raising £1m for Sport Relief. He later received a special award at the BBC's Sports Personality of the Year for completing the 21-mile distance.
He swam the Strait of Gibraltar from Spain to Morocco for Sport Relief in 2008, and last year cycled from John O'Groats to Land's End.
In his latest challenge the comedian is again raising money for the BBC-backed charity, which helps vulnerable people in the UK and in some of the world's poorest countries.
"Most people in the UK know the Thames, and most people think the idea of swimming in it is horrifying," Walliams said of the challenge.
"And it is. It'll be relentless. It's a huge distance so I'll need all the support I can get."
To support Walliams visit his pages on the Sport Relief website.
To learn how to swim, or how to improve technique, visit the Big Splash website.
Walliams vs The Thames will be filmed for a documentary to be broadcast in the build up to Sport Relief in March next year.