Captain Guyan March, from Cornwall, is among 31 crew missing on board the Fantome. It lost satellite telephone contact eight days ago as it headed from Belize to Roatan to try to outrun the storm, which has already killed more than 7,000 in Honduras and Nicaragua.
Search parties are keeping an open mind as to the fate of the crew, but the US Coast Guard fears the boat - a four-masted craft formerly owned by the shipping tycoon Aristotle Onassis - has sunk. Sea searches around the islands of Guanaja and Roatan, off Honduras, discovered debris, seven life-jackets and two empty life rafts from the Fantome.
Rescuers are being helped in their search by a Royal Navy warship, HMS Sheffield. The Foreign Office confirmed last night that one Briton was among those missing.
The yacht is registered in Antigua and is operated by Miami-based Windjammer Barefoot Cruises. It was on a tourist cruise near Belize and the Bay Islands, which was abandoned as Mitch approached. The crew dropped off 100 passengers on shore on Monday last week before sailing towards Roatan to avoid the storm.
Coastguards said 11 crew members were from Guanaja, four each from Grenada and St Vincent, three from Jamaica, two from Honduras and one each from Nicaragua, St Lucia, Panama, Trinidad, Antigua, Romania and England.
Two US Coast Guard spotter aircraft and a helicopter from HMS Sheffield have been used in the search effort and have so far found debris at Guanaja and north of Roatan.
Among the wreckage was part of the Fantome's wooden staircase. The serial number on one of the life rafts was confirmed as that of the Fantome.
More than 120,000 square miles have been scoured since Thursday evening in the search for survivors. Several people who travelled as holidaymakers on board the Fantome have set up Internet sites to exchange news about the missing craft.
Laura Marshall, from Atlanta, Georgia, said: "Captain Guyan has been sailing since he was a kid. He is a fantastic sailor. He knew what he was doing. He did the right thing."
A spokeswoman for Windjammer Barefoot, which has refused to confirm Captain March was among those missing, said the discovery of the debris from the Fantome did not necessarily mean it had sunk.
She said: "There are things on the ship that can be dislodged with a wave that came across the bow. That would explain some items being in the ocean. Of course we are concerned, but we still feel the ship is out there and are continuing to search for it."
But Scott Carr, a spokesman for the US Coast Guard, said: "We have found all sorts of stuff. It is starting to look as if the ship went down."
The SV Fantome, at 282ft, the world's largest four-masted stay-sail schooner, was originally built as a destroyer for the Italian Navy, but the only service it ever saw was that of plaything for the rich and famous.
The Duke of Westminster bought it in 1927 and christened it the Flying Cloud, but it quickly passed into the hands of an American who repeatedly sailed round the Mediterranean with two sea-sick cows so he could enjoy fresh milk.
The Guinness dynasty next enjoyed its pleasures, renaming the vessel, before it was bought by Aristotle Onassis - second husband of Jackie Kennedy - who intended the ship as a wedding present for Prince Rainier of Monaco when he married the film star Grace Kelly in 1956. He kept it for himself when he failed to receive a wedding invitation.
It lay rusting in the north German harbour of Kiel until 1969, when Mike Burker, found- er of the Miami-based Windjammer fleet of holiday yachts, restored the dilapidated steel-hulled ship with a $6m (pounds 3.6m) refit. The yacht can take up to 128 passengers and 45 crew.