Adelaide-based sailor presumed lost at sea

An Adelaide-based British crewman taking part in the Volvo Ocean Race is presumed lost at sea after falling overboard in the Southern Ocean.

Adelaide-based British yachtsman John Fisher was swept overboard during a round-the-world race. (AAP)

An Adelaide-based British sailor who went overboard during a round-the-world race in the Atlantic Ocean is now presumed to have been lost at sea, competition organisers have said.

President of the Volvo Ocean Race Richard Brisius said John Fisher, a crew member of the Sun Hung Kai/Scallywag yacht team, was thought to have perished in the "extreme" sea conditions around 1400 nautical miles west of Cape Horn, South America.

Brisius said despite conducting an exhaustive search in challenging conditions, teammates had been forced to turn towards land without him.

"Given the cold water temperature and the extreme sea state, along with the time that has now passed since he went overboard, we must now presume that John has been lost at sea.

"All of us here at the Volvo Ocean Race organisation send our heartfelt condolences out to John's family, his friends and his teammates and we will do everything in our power to support them in this very difficult time."

Brisius said the Sun Hung Kai team remained in a "challenging position" as weather continued to deteriorate throughout the day, and crew members had been left "emotionally and physically drained" by their friend's loss.

"Our sole focus now is to provide all the support and assistance that we can to the team," he added.