The testimony casts fresh doubt on the controversial trial of migrant workers Zaw Lin and Win Zaw Tun who are accused of the murder of 24-year-old David Miller and the rape and murder of Hannah Witheridge, 23, on southern Koh Tao island in September 2014.
Both men have pleaded not guilty and face the death penalty if convicted over a case which has tarnished Thailand's reputation as a tourist paradise and seen the police accused of bungling the investigation.
"The DNA on the weapon did not match with the two suspects," Porntip Rojanasunan, director-general of the CIFS, told AFP after testifying at Koh Samui Provincial Court today.
We found the DNA of two "unidentified people" on the hoe, she added.
The battered bodies of Miller and Witheridge were found on the sleepy diving island of Koh Tao on September 15.
Police say Miller had been struck by a single blow and left to drown in shallow surf while Witheridge had been raped and then beaten to death with a garden hoe.
Nakhon Chomphuchat, the lead lawyer for the migrants, said today's testimony showed the two men "were not involved with the case as police have accused".
Porntip also told the court that there was no DNA found on other items tested by the forensics institute including a shoe and some plastic bags.
In July a witness at the trial had testified to removing and washing down the garden hoe after coming across it shortly after the tourists' bodies were found on the island.
