The record of the Channel crossings is held by Alison Streeter, who managed to make 43 successful swims across the channel.

British star David Walliams, well known actor and writer had the initiative of crossing the English Channel for charity. The purpose of this attempt is to raise money for Sport Relief. The comedian is aware of the difficulty of his attempt and that is why he trained himself for almost 12 months with former Olympic pentathlete Greg Whyte.

Walliams is saying that he will "just be in a pair of Speedos and goggles - it's what the public wants," but he took his training very seriously. The crossing of the Channel is known as "the Everest of open-water swimming" and only a few of the ones that try it succeed.

But jokes aside, what the 34-year-old comedian is attempting is quite a feat. Few people succeed - just 10% according to some figures - and the swim is known Walliams, who is raising money for Sport Relief, has spent the last 11 months training and Alison Streeter, the record holder for the Channel crossings explains what are the most difficult things is this attempt:

"It's not about the distance, lots of people can swim the equivalent in a pool," she says. "It comes down to the cold. Fit swimmers are often all muscle and don't have much fat on them, this means they get cold quicker and once the cold gets into your muscles it is very hard to continue."

Considering that Walliams plans to make his attempt some time between 28 June and 15 July which is early in the season, it means that the cold will be an even greater issue.

Other important factors that make the attempt difficult are the tides, the wind and the weather. The tides are hard to predict as they are strong and change direction approximately every six hours and they change in height and flow speed every day.

The local weather conditions can change very quickly and sometimes do not match the forecasts.

A problem not to be ignored is the large number of ships using these waters, as to go from England to France you have to swim across the shipping lanes. There are more than 600 commercial ship movements and almost 100 ferry crossings between Dover and Calais every day. The difficult job of keeping the swimmer out of the path of these vessels, it’s the job of the accompanying pilot boat.

Ms Streeter says that she has “done 43 swims - seven in one year once - and every one of them has been different" but she believes that "David Walliams seems to be preparing in the right way, so he will have a fair chance."

On 4th July 2006, David Walliams started from a beach near Dover and successfully swam the 21 miles across the English Channel in 10 hours 30 minutes. Before the swim he said he hopes to complete the 21-mile charity swim in less than 16 hours and declared: "I reckon I have a 50/50 chance of getting across. I hope people see it as a symbolic thing".