Media playback is unsupported on your device Media caption Doubt has been cast over whether DNA evidence can be re-tested, Jonah Fisher reports from Koh Samui

Crucial DNA evidence in the case of the murder of two British backpackers cannot be retested because it no longer exists, Thai police have told the BBC.

The evidence is central to the prosecution case against two Burmese migrants on trial for killing David Miller, 24, from Jersey, and Hannah Witheridge, 23, from Norfolk.

The defence had wanted it re-examined.

It comes as Zaw Lin and Wai Phyo, who both deny murder, rape and robbery, appear in court for a second day.

The judge in the trial had been due to make a decision on whether the DNA evidence could be independently verified.

Police tests

The tourists' bodies were found on a beach on the island of Koh Tao last September, sparking a major police investigation and intense local and international pressure to find those responsible.

Thai police claim DNA they gathered and tested in a police laboratory links both men to Miss Witheridge's body.

Speaking to the BBC, Lieutenant Colonel Somsak, who led the original investigation, said some of the original DNA samples had been "used up".

A hair sample found in Miss Witheridge's hand was among samples that were lost, he said.

Lieutenant Colonel Somsak said all police could offer the court was documentation of the results.

"There's nothing left. It was used up when we tested the first time," he said.

Image copyright AFP/getty images Image caption Zaw Lin, front, and Wai Phyo (also known as Win Zaw Htun), rear, arrive at the court in Koh Samui

BBC Myanmar/Burma correspondent Jonah Fisher, who is in Koh Samui for the trial, said the DNA evidence had seemed like the strongest piece of evidence in a complex and confusing case - and now it could not be independently verified.

The news would support the argument of those who believed the accused had been framed by the Thai authorities, our correspondent said.

Bodies 'unclothed'

On the opening day of the trial the first police officer to reach the murder scene described how the bodies were found a few metres apart, and were unclothed.

Mr Zaw and Mr Wai (also known as Win Zaw Htun), both 22, were arrested several weeks later.

They are said to have confessed to the crime but later retracted their statements.

The defendants have repeatedly stated their innocence over the murders, which happened on 15 September last year, with the defence alleging they were framed.

Verdicts in the case are not expected until October.