The families of two British backpackers murdered on a Thai island last year will attend the trial of the alleged killers, they said Wednesday, in a grim case that has tarnished the country's tourist haven reputation.

Zaw Lin and Win Zaw Tun -- both Myanmar nationals -- are accused of murdering 24-year-old David Miller and the rape and murder of Hannah Witheridge, 23, on the southern island of Koh Tao in September.

Their high profile trial kicks off later Wednesday on the neighbouring island of Koh Samui, where they face several charges including murder, rape and robbery. If found guilty they could face the death penalty.

The prosecution has been marred by allegations of a bungled investigation and claims from the defence team that the migrant workers, who both worked for low wages in the tourism trade, have been scapegoated by an under-pressure police force.

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, a 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 their attendance at an 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.

Bright future ended

"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."

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 also complain they have not been given access to the forensic evidence, despite the court initially ruling in April that they could run their own independent tests.

But in late May they were told a final decision on the forensic evidence would only be made on the first day of the trial.

"There's no reason why this information should be withheld from the defence team," Andy Hall, an activist with the Migrant Worker Rights Network, which is helping to fund the pair's defence, told AFP.

"There's a real lack of adequate disclosure by the prosecution and that worries us about whether there will be a fair trial," he added.

Nonetheless Thai police and prosecutors insist they have charged the right men, saying the forensic evidence 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.