Double Olympic sailing champion Andrew 'Bart' Simpson, 36, had been sailing on the Artemis, Sweden's entry in the America's Cup, when the catamaran capsized, according to a statement for the team posted on the Artemis racing website.
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The 36-year-old has not been seen or heard from since Thursday evening.
Sophie Neville, cast as Titty as a young child in the 1974 classic Swallows and Amazons, is advising its modern-day adaptation.
At least two people are feared dead in the attack which is the latest on medical facilities in the war-torn country.