PRESS ASSOCIATION -- David Walliams has set off on his most dangerous challenge so far - swimming the entire length of the Thames.
The comedian and panel show host is raising money for Sport Relief by swimming 140 miles over eight days from Gloucestershire to London.
Hundreds of well wishers lined the river bank at Lechlade to give the Little Britain star a loud send off and before taking the plunge into the icy river, he spoke to many of them.
The comedian said that when the swim got really tough, he would think about how the money he raised would help some of the poorest people in the world.
"I think about happy things, I think about what I'm going to eat when I get out," he said.
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"I think about songs and I have been on some trips with Comic Relief and Sport Relief and I think about the people the money raised will help."
David described himself as a "masochist" for wanting to do another swimming challenge.
"I must be a masochist. I wanted to do something else and I'd just turned 40 and I thought I haven't got much time left because my body is falling apart," he said.
He said this swim would be 120 miles longer than swimming the English Channel. "That's the scary thing. It's all right to be full of bravado today but this is day one of what will probably be eight days of swimming," he said.
The BT Sport Relief Challenge: Walliams vs The Thames will be filmed for a documentary to be broadcast in the build up to the Sport Relief weekend in March next year. People can check out the GPS tracker online and follow the Twitter feed using hash tag #Thamesswim to see where he will be.