A British yacht owner was found dead in his boat on the Thai island of Phuket.

Graham Patrick Davis Tucker, 69, who also had Australian nationality, had not been seen in several days before he was discovered lifeless in his ship anchored off Khao Khad Beach on January 11.

Police boarded the Synergy Sydney vessel, moored opposite the Veranda Resort Phuket hotel, and found Graham lying in a bedroom at the bow.

They said the body was 'significantly decomposed' with purplish-black discolouration, peeling skin, and lymphatic discharge. He is believed to have been dead for up to two days.

Investigators said there were no signs of a struggle or ransacking at the scene.

Police Lieutenant Watcharakorn Suwan of Wichit Police Station said: ‘Regarding the reported news, we were informed by a hotel employee. We are currently investigating, gathering evidence, and awaiting autopsy results to determine the cause of death. We have not ruled out the possibility of an unnatural death.'

Staff said Graham was not a guest at the hotel.

The gruesome discovery was made when Graham's German friend, Matthias Lepke, 54, whose yacht was anchored nearby, became concerned over his prolonged absence.

Matthias checked the 'unusually quiet' yacht before finding his friend's corpse at around 12:30 pm local time. He immediately alerted staff at the Veranda Resort.

Locals said Graham, who holds a British and Australian passport, had been living on his yacht for around a decade, travelling back and forth between Malaysia and Phuket.

Police said they have collected evidence from the scene and sent Graham's body to the Vachira Phuket Hospital for a post-mortem exam.

The British and Australian embassies have also been notified to inform the relatives of the deceased.