On his Twitter profile he described himself as an "Endurance athlete, iron & ultraman triathlon for many years".
One of his last posts on the social media site was earlier in August before he went for a swim.
A spokesman for the governing body, the Channel Swimming and Piloting Federation, confirmed a man had died.
He said: "A swimmer was taken from the water less than a mile from the finish in France.
"There was an extremely experienced support team on board the support vessel. He was given CPR and taken to Calais where he was sadly pronounced dead.
"It is an extreme sport, we know the risks. He was doing what he loved doing. Our thoughts are with his family."
Enduroman Ultra Events, which was tweeting his solo progress since leaving the UK early on Saturday, wrote: "Our friend Nick Thomas left us whilst doing what he loved - he just kept going.
"He'll always be a part of what we do and who we are."
The Enduroman event combines running, cycling and swimming. It begins with an 87-mile run from London to Dover, followed by swimming the English Channel and is supposed to end with a bike ride to Paris.
The organisation raised the alarm on their Twitter feed on Saturday night, posting: "Nick Thomas is being treated at Calais hospital after passing out in the water. Please wait for an update."
A previous message had said: "Flood tide is strong pulling us toward Blanc Nez, hopefully some chance to land near there. Tough going for Nick."
A Foreign Office spokesman said: "We are offering assistance to the family of a British national following his death in Calais, France."
