Police accompany Linda Robertson back to land, along with the three suspects
A British man was murdered and his body thrown overboard by three men who then forced his wife to steer the boat to shore, police in Thailand said.
Malcolm Robertson, from East Sussex, and his wife Linda were attacked off Satun, the Foreign Office confirmed.
Thai Police have arrested three Burmese men in connection with the incident.
Police Captain Suparak Pongkarnjana said Mr Robertson shouted for help but the men "slit his throat with a knife and hit him".
Linda and Malcolm Robertson were sailing near the island of Koh Dong
So far no body has been found but it is understood the 64-year-old was killed on Monday while sailing with his wife near Koh Dong, an island about 45 miles west of Satun.
Three migrant workers from Burma are said to have confessed to killing Mr Roberston when he caught them trying to steal a dinghy from his moored yacht.
Mrs Robertson's brother, John Clee, said the couple, from Hastings, were sleeping in different parts of the boat in the early hours when people were heard on board.
Mr Clee, who spoke to his sister by telephone on Tuesday, said: "She heard people coming on board the boat and called out to Malcolm to let him know that someone was coming in.
"Malcolm must have come up and spoken to them because she heard him - there was a confrontation and then she heard him say 'get off the boat, get off the boat'.
"There was a bit more kerfuffle and nothing else was said, and it all went very quiet."
He said the people came into her cabin and tied her up before taking her up on deck so she could help them sail the yacht.
He said: "As she came out of her cabin she said it was a nightmare - there was just blood everywhere.
"She knew then that something awful had happened to Malcolm and she said then that she knew he was dead."
Brother tells of yachtswoman's 'traumatic' Thai ordeal
Mr Clee, 63, from Battle, said the intruders sailed the yacht for hours before making off in a dinghy, at which point his sister sailed the boat to a nearby fishing vessel where she found help and the police were called.
He said the pair were qualified yacht masters who had sailed around the world and it had been Mr Robertson's life-long dream to spend the winter months sailing with his wife.
Mr Robertson was the owner of a chain of coffee shops around Hastings, but was semi-retired, he added.
The couple, who had been married for 25 years, had two children each from previous relationships and seven grandchildren between them.
All four children were on their way to Thailand to support Mrs Robertson, who escaped with minor injuries, he said.
A spokeswoman for the Foreign Office said: "We can confirm that a British couple were attacked whilst sailing of the coast of Satun, southern Thailand last night.
"Our consular team in Bangkok are in touch with the next of kin and are providing consular assistance to those involved.
"We are urgently pursuing this case with local police who are investigating the incident. We understand that the Thai police have now arrested three suspects.
"One British national has been reported missing and one British national is in hospital."
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