The high-profile trial of two Myanmar migrants charged with killing two British holidaymakers opened Wednesday on the holiday island of Koh Samui, in a grim case that has tarnished Thailand's reputation as a tourist haven.

Zaw Lin and Win Zaw Tun have pleaded not guilty to the murder of 24-year-old David Miller and the rape and murder of Hannah Witheridge, 23, on the southern island of Koh Tao in September.

The two men, who have been in custody on neighbouring Koh Samui since October, arrived at court in a prison van with their feet shackled. They face several charges -- including murder, rape and robbery -- and if found guilty they could face the death penalty.

Family members of the two British backpackers were also present for the opening of the trial, as the prosecution remains marred by allegations of a bungled investigation.

The defence team claim the migrant workers, who both worked for low wages in the tourism trade, have been scapegoated by an under-pressure police force as the murders put Thai authorities in the international spotlight.

After the trial started Wednesday Andy Hall, an activist for a migrants group helping to fund the pair's case, told AFP the judge will decide whether the defence can independently test the controversial forensic evidence against their clients on Thursday.

"We feel confident he will allow it," he said of tests the defence have been demanding for months in order for a "fair trial".

'Bright future ended'

The lifeless, battered bodies of Miller and Witheridge were discovered on a beach just a few hundred yards from the main tourist drag in Koh Tao, sending shockwaves across the beach-fringed idyll in the Gulf of Thailand popular with backpackers and divers.

Miller's beaten body was found in shallow surf while Witheridge's was located slightly further up the beach. Police say she was raped as well as beaten.

The families of both victims released statements early Wednesday confirming they would attend the trial's opening at the imposing courthouse perched on a hill overlooking Samui's lush palm trees and white beaches.

"Just hours before he died David was talking to us with his usual enthusiasm, describing the beauty of Koh Tao and the friendliness of the Thai people," Miller's family said in their statement, adding that they hoped to "gain a better understanding" of how the young Brit died.

"Hannah was a beautiful person, inside and out, she brought a room alive just being there," the Witheridge family wrote in their statement.

"Her bright future was brutally ended leaving those who loved her broken with no answers."

Miller's parents and brother were present at court Wednesday as well as Witheridge's father and brother.

Both families appealed for privacy from the press for the duration of the trial, which is expected to take place over 18 staggered days between now and September with a verdict due in October.

The killings came as the country's vital tourism industry was beginning to recover from months of violent street protests that culminated in the May 2014 military coup.

The case also shone a light on Thailand's many underpaid and often exploited Myanmar migrant workers who fill the lucrative tourist sector.

The pair's defence team have long criticised the police investigation, claiming the crime scene was contaminated and that their clients were tortured into admitting guilt.

Both men retracted their initial confessions, saying they were coerced into making them.

The defence team had also complained about not being given access to the forensic evidence, despite the court initially ruling in April that they could run their own independent tests.

"There's a real lack of adequate disclosure by the prosecution and that worries us about whether there will be a fair trial," Hall, from the Migrant Worker Rights Network, told AFP before the judge confirming a decision on this would be reached Thursday.

The forensic material from the crime scene has been used by Thai police and prosecutors to insist they have charged the right men, saying it strongly points to the Myanmar pair as the perpetrators.

The victims' families have also previously said they have confidence in the case after British investigators reported back to them following a visit to Thailand late last year.