Cracknell (L) and Walliams will swim 12 miles together

The champion rower will cycle, row and swim from the UK to Africa, with Walliams joining him for the swimming leg of the challenge next month.

In 2006 Walliams raised over £1m for the charity by swimming across the English Channel.

"I'm looking forward to getting back into the sea," said the 36-year-old.

'Challenging swim'

Walliams had been due to be in the US working with his fellow Little Britain partner Matt Lucas but said he became available after the writer's strike finished last week.

"I thought, 'what could I do?' And I thought perhaps I could steal some of James Cracknell's thunder by joining him on the swimming part," he said.

Perhaps I could steal some of James Cracknell's thunder by joining him on the swimming part David Walliams on why he wanted to take part

"People might think that I did all the rest of it too," he joked.

The star, who plans to swim the 12-mile stretch of the Strait of Gibraltar, said although the distance is shorter than the English Channel there will be other problems to face.

"It's colder and also there's sharks in it and also the sea's a lot rougher, so I think it's going to be a very challenging swim," he said.

Ahead of the task, the pair trained by taking a icy dip at Dorney Lake in Windsor.

"We hyperventilated when we first got in the water because it was so cold and after that your whole body just goes numb," said Cracknell.

He is hoping to set off from the UK on 28 February and will row to mainland Europe before getting on his bike and pedalling through Spain and France.

It is thought he will cover more than 1,400 miles.