The trial of two Burmese migrant workers charged with the murder of British backpackers, David Miller and Hannah Witheridge on the island of Koh Tao in Thailand last September has taken another twist.

Thai police have told the BBC that they are unable to produce the DNA samples collected at the murder scene of Miler and Witheridge, claiming that they are either missing or have been used up in the testing process.

The defence team is seeking to have forensic evidence collected at the murder scene to be re-tested independently.

Zaw Lin and Wai Phyo, have both pleaded not guilty to the murders, rape and robbery. They claim they were forced to confess to the murders and have since withdrawn their confessions.

The judge in the trial is due to decide on Friday (10 July) whether the DNA evidence collected could be independently verified as requested by the defence team.

Lead defence lawyer Nakhon Chompuchat said the police will later confirm Friday (7 July) what evidence they have in their possession.

Speaking to the BBC, Lieutenant Colonel Somsak, who led the initial investigation said some of the original DNA samples had been 'used up' while a hair sample found in Witheridge's hand was among the samples that were lost.

All the police could offer the court was the documentation of the results of the DNA samples, Somsak said.

"There's nothing left. It was used up when we tested the first time," he told the BBC. The DNA evidence collected and tested in a police laboratory links both the defendants to Witheridge's body, the Thai police have claimed.

Bangkok Post said that Somsak Nurod, the chief investigator at the Phangan police station appeared before the court to discuss the defence team's request for re-testing of forensic evidence collected. He is expected to inform the court on Friday about the status of the evidence kept by the police at the station.

He told the court that some items had already been taken during the forensic checking process. They include three cigarette butts and a condom. The Thai police also had about 300 cotton buds with DNA samples of the defendants.

A shovel, sandals, and sand soaked with blood remain at the Phangan station, he said.

Outside the court, he told reporters: "The DNA examination of the cigarette butts is complete. Of course the items are kept appropriately. But the DNA samples taken from the bodies are not my responsibility. They are in Bangkok."

On the first day of the trial, Nakhon revealed that the legal team had received information from the British authorities that shows inconsistencies in the prosecution case.

Although he did not provide further details, the Telegraph said the evidence is believed to have come from the examination of the bodies of the murder victims in the UK and is related to DNA found on Witheridge's body.