A Thai court in Koh Samui sentenced two Myanmar migrant workers to death today after convicting them of the 2014 murders of two British tourists on a holiday island in a case mired in controversy.

The battered bodies of backpackers Hannah Witheridge and David Miller were found on a beach on the island of Koh Tao in September 2014.

Following weeks of pressure to solve the case, police arrested Myanmar migrant workers Zaw Lin and Win Zaw Tun and later said the two had confessed to the crimes.

The verdict and sentence follow an investigation and trial that triggered allegations of police incompetence, mishandling of evidence and torture of the suspects. Both later retracted their confessions, saying they had been made under duress.

The long-awaited verdicts came after 21 days of witness hearings in a trial that began in July and ended in October.

Allegations by defence lawyers of police incompetence and evidence mishandling dominated the trial.

As is customary in Thailand, where trials have no jury, a judge delivered the verdict and sentence and said the DNA tests by investigators were carried out to accepted standards and the DNA found on Witheridge matched that of the defendants.

The judge said there was no weight to the two men's claims that they had been tortured.

Miller's family flew to Thailand for the verdict. His brother, Michael Miller, delivered a statement to reporters outside the court and said justice had been delivered, adding that the two men had shown no remorse for what they had done.

"We believe what happened today represents justice for Hannah and David," said Miller.

"The Royal Thai Police conducted a thorough and methodical investigation ... evidence against the two was overwhelming."

The mother of one of the defendants broke down in tears as the judge passed sentence in the court on Samui island, close to Koh Tao.

The defence said the two men would appeal.

Andy Hall, a Thailand-based activist for the rights of migrant workers, said the defence team was surprised by the sentence and that volunteers would continue to provide humanitarian assistance to the two men.

The brutality of the murders dented Thailand's image as a happy-go-lucky holiday paradise and raised serious questions about its treatment of migrant workers.

Police were widely accused of bungling the investigation, including failing to close off the island quickly and allowing potential suspects to escape.

Police said Witheridge was found bludgeoned to death and had been raped. Miller also suffered blows to his head.

A debate over DNA samples that police say link the two suspects to Witheridge's body has been at the heart of the trial.

Defence lawyers had asked to retest crucial DNA samples taken from the bodies but authorities issued conflicting statements on DNA evidence and, at one point, said that it had been used up.

No independent re-testing of DNA evidence has been done in the case.

In his closing remarks to reporters, Miller said the family's life had changed forever.

"The love we have for David will never be taken away. We remain so proud of him."

- Reuters