Thai police have charged three people over the attack on a British couple on their yacht in southern Thailand.

Linda and Malcom Robertson were on vacation off the southern province of Satun, which borders Malaysia, when their yacht was attacked.

Malcom Robertson's throat was slit and his body thrown overboard, according to police captain Suparak Pongkarnjana.

The 64-year-old's body has not yet been found.

The three migrant workers were arrested as they tried to flee in the dinghy taken from the Robertson's yacht.

They have been charged with aggravated assault and illegal migrant worker status.

"We are searching for the body," said the head of La-ngoo police station. "If we find the body then the three suspects will be charged with murder. We have imposed charges of illegal migrant workers and aggravated assault against the suspects first."

During the ordeal Linda Robertson, 57, was apparently tied up naked like a "trussed chicken" and held hostage in the cabin for up to 10 hours.

Her brother, John Clee, who has spoken to her since the incident, said the robbers took her up on deck from time to time when they needed help sailing the 44-foot yacht.

At one point she remembers "walking in my husband's blood" on the deck.

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