The naked bodies of a 23-year-old woman, from Great Yarmouth in Norfolk, and 24-year-old man from Jersey in the Channel Islands, were discovered this morning on Sai Ree beach, around 100 metres away from the scene of a moonlit beach party.
A blood-stained hoe, found near the bodies, is thought to have been the murder weapon.
Police colonel Prachum Ruangthong said: "The man was chopped in the back and on the side of his head, while the woman was chopped in her face.
"It's very gruesome."
The Foreign Office released the following statement:
"We are aware of the deaths of two British nationals on September 15 on the island of Koh Tao in the Gulf of Thailand.
"The embassy is urgently seeking information from the local authorities and consular staff are ready to provide assistance to the friends and family at this tragic time."
It's thought the attackers may have been waiting to target the man and woman as they walked off at the end of the full moon party, which was attended by a group of tourists last night.
The island has now been cordoned off, the pier blocked and ferry routes to and from the mainland suspended, but it's thought that those responsible may have already fled.
Stock images used.