Two men convicted of the murders of two backpackers in Thailand will be executed after a court threw out their final appeal - despite serious concerns they were tortured into confessing to the brutal crime.

Their appeal was rejected by Thailand's supreme court on Thursday.

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The badly battered bodies of tourists David Miller, 24, and Hannah Witheridge, 23, were found on a beach on the island of Koh Tao in the Gulf of Thailand early on the morning of September 15, 2014.

Miller had shocking head injuries, and Witheridge had been raped and beaten to death.

Thai police investigate the murders in 2014. Credit: Sakchai Lalit / AP

Both were killed with a gardening hoe.

The British backpackers had gone to Thailand separately and met at the hotel where both were staying.

Their convicted killers, Myanmar migrant workers Wai Phyo and Zaw Lin, were employed as service workers on the island, which is famous for its stunning snorkelling and diving locations.

Doubt grows

However, doubt has since swirled around the validity of Phyo and Lin's confessions and police competence has been called into question.

Lawyers for the two convicted men claimed the evidence in the case was mishandled and they had made their confessions under duress.

But at their trial, a well-known Thai forensics expert testified that the prosecution's DNA evidence did not link the defendants to the scene.

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The expert also alleged that police had failed to properly control the crime scene and mishandled the DNA evidence.

It is believed the pair is being used as scapegoats for the murders.

Myanmar migrant workers Wai Phyo and Zaw Lin face the death penalty. Credit: Sakchai Lalit / AP Photo

But the court rejected the defence arguments and in December 2015 convicted both defendants of murder and sentenced them to death.

At the time, Human Rights Watch said the verdict was "profoundly disturbing", citing the defendants' accusations of police torture that were never investigated and questionable DNA evidence linking them to the crime.

Appeals fail

In a 2017 appeal, the guilty verdict was upheld, and now the final appeal has now also affirmed the conviction.

The men's lawyers said they would be seeking a royal pardon for the pair.

If that fails, they face death by lethal injection.

Adviser to the defence team, Andy Hall, said the evidence was not reliable and the pair should be exonerated.

Chevron Right Icon '(The evidence was) fundamentally flawed.'

He said the forensics evidence was "in my opinion fundamentally flawed and should always have been considered unreliable when considered against international standards on DNA and forensics usage in criminal trials".

"I express my deepest sympathy to all those whose lives have been touched and changed forever by this tragic case," he said.

While there had been a pause in the use of the death penalty in Thailand since 2009, that ended in June last year when a man was executed by lethal injection for murdering another man for his phone.

- with AP