KOH SAMUI (REUTERS) - Thai police said on Thursday some key DNA evidence in the trial of two men for the murders of two British tourists had been lost or "finished" and so could not be retested as the defence has demanded.

The trial of two Myanmar migrant workers, Zaw Lin and Win Zaw Htun began on Wednesday. The two men are accused of killing David Miller, 24, and Hannah Witheridge, 23.

The high-profile case has raised questions about the competence of the investigation and the treatment of migrant labourers in Thailand.

The murdered pair were found on a beach on Koh Tao, an island in southern Thailand popular with backpackers and scuba divers, on Sept 15.

Post-mortem examinations showed both had suffered severe head wounds and Witheridge had been raped.

Both accused men deny the killings.

The defence wants some evidence central to the case re-examined. But Police Lieutenant Colonel Somsak Nurod, who led the original investigation, said some evidence could no longer be re-tested as it had been lost, including a hair sample found in Witheridge's hand. "There is documentation of the testing we did at the time but some of the evidence, including the hair sample, was lost, so it cannot be retested," Somsak told Reuters in a telephone interview.

He added that biological samples found on cigarette butts taken from the crime scene were "finished", but declined to go into further details.

Chief defence lawyer Nakhon Chompuchat told reporters that a garden hoe allegedly used as the murder weapon would be retested.

"We contacted the institute and they said if they are doing the testing as an emergency case then it can be done in three to seven days," said Nakhon. "We will definitely test the garden hoe."

Cross-examination on Wednesday centred on why police were slow to seal off the crime scene, why doctors arrived hours after the bodies were found and whether officers on the island were trained to use a DNA testing kit.

Thai police said in October the two accused men had confessed to the killings, but they later retracted their confessions, saying they had made them while being tortured.

Police say they have a watertight case, and that DNA found on the victims matched the suspects.

Critics have argued the arrests resulted from discriminatory ethnic profiling.

The men could face the death penalty if found guilty. A verdict is expected in October.