Steve Redmond was forced to abandon his attempt to become the first person to swim from Ireland to Wales in the early hours of this morning.
He set off from Carne Pier in Wexford last night, but ended his attempt on the advice of his support team.
John Kearney, from Mr Redmond's support team, said the record-breaking swimmer was advised to throw the towel in by medics just hours after setting off.
"I know many people will be disappointed, not least Steve himself, but we made the right decision to end the attempt," he said.
"Our main focus and concern was always Steve's health and when he started displaying worrying signs of hypothermia we felt that with such a long swim ahead of him it was not worth the risk.
"Steve has swam some of the most gruelling oceans and he knows that everything has to be right and you don't take chances."
Mr Redmond hit the headlines last year for becoming the first person to complete the Oceans Seven Challenge.
This involved swimming the English Channel, North Channel (Northern Ireland - Scotland), Strait of Gibraltar, Catalina Channel (California), Cook Strait (New Zealand), Molokai Channel (Hawaii) and the Tsugaru Channel, Japan.
He had vowed never to take on another endurance swim again, but decided to swim from the Irish Sea to raise money for the RNLI.
Officials from his team said while he was recovering well, it is too early to say whether he intends to give the swim another go.