The court hearing the trial of two Burmese migrant workers charged with the murder of British backpackers David Miller and Hannah Witheridge has ordered the re-testing of forensic evidence collected at the murder scene.

The Samui Provincial Court, in allowing the defence team's request, ordered the public prosecutors to work with investigators to send all remaining forensic evidence, including a shovel, to the Justice Ministry's Central Institute of Forensic Science for re-testing.

Zaw Lin, 22 and Wai Phyo, 21, pleaded not guilty to the murders, rape and robbery of Witheridge and Miller on the island of Koh Tao. They claim that they were forced to confess to the murders and have since withdrawn their confessions.

The defence team, appointed by the Lawyers' Council of Thailand, has said that it had received forensic and autopsy reports from the UK authorities that were not consistent with the findings of the Thai authorities.

On claims that forensic evidence was missing, National Police Chief Somyot Poompunmuang clarified that the forensic evidence were no longer in the custody of police as they had been sent to the Institute of Forensic Medicine.

He said the re-examination of the DNA evidence is still possible, Bangkok Post reported.

Earlier, police doctor Pawat Prateepwitsurut, who conducted the autopsies on the victims after their bodies were flown to Bangkok on 15 September 2014, told the court that Witheridge had been sexually assaulted and died of severe head injuries caused by an instrument with a sharp blade.

He said that he also found a hair in Witheridge's hand that he thought was blonde. "It was sent to the lab to be tested, but they were unable to establish its colour. To my eyes, it was blonde," he said, according to The Telegraph.

Miller had cuts on his arms, possibly as a result of trying to defend himself. He had received a number of blows to the head, while scratches on his arms suggested that he was dragged along the beach. Dr Pawat said water found in Miller's lungs indicated his death was caused by drowning.

The trial has been adjourned to 22 July.

Miller family visits scene of murder

The family of David Miller travelled to the Koh Tao island last Sunday where their son was killed to create a personal shrine in his memory.

They left a photo of David, a message, a programme from his funeral service and a candle on the rocks above Sairee Beach where his body was discovered.

"Part of the reason for coming out was to go to Koh Tao. We had one day on Koh Tao. It was really hard," David's mother said. His brother said: "I can't describe how I feel."

According to the Telegraph, the family members had asked for their first names not to be used.

The family is unsure if they will attend the entire trial, which is scheduled to end on 25 September.

"We are going to keep in touch with the whole process, depending on work commitments. Whether we're here or not, we'll be listening to it," Mr Miller said.