BANGKOK - Police originally charged the two migrant workers with the murder of David Miller, 24, and the rape and murder of Hannah Witheridge, 23, in October, citing DNA tests and later confessions by the pair to the crime on the diving island of Koh Tao.
But Zaw Lin and Win Zaw Tun, both in their 20s, later retracted their confessions, alleging they were obtained under duress, raising concerns over the probity of the investigation.
Prosecutors on nearby Koh Samui, where the pair are being held, had for several weeks repeatedly rejected the police file on the case, saying it was incomplete.
But on Wednesday they agreed to finally indict the suspects for trial.
"They (the two suspects) were charged with intentional murder," Paiboon Archavanuntakun, Koh Samui's chief prosecutor, told AFP.
"Tomorrow afternoon (Thursday), the order will be submitted to the Koh Samui provincial court," he said. "There will then be a trial," he added, without giving details of a timeframe.
The start of court proceedings will come as a blow to the men and their families, who have made impassioned appeals in Myanmar and Thailand for their release.
Rights campaigners in Thailand insist the pair have been made scapegoats for a vicious crime which appeared to initially stretch the capacity of police.
The murders, which occurred in September on a normally tranquil island, also cast a long shadow over the nation's reputation as a tourist haven, prompting the Thai junta leader Prayuth Chan-ocha to vow to swiftly bring the killers to justice.
Alarmed by the initial handling of the probe - which saw reporters allowed to trample all over the crime scene among a slew of apparent bungles - a team of British detectives visited Thailand in November to review the investigation.
They are yet to reveal their findings.
The accused have continued to protest their innocence, submitting a letter on Tuesday to the court urging witnesses come forward to help clear their name.
"This will really assist us in our defence and ensure justice is done for us, our family and the family of the victims," the letter signed by both men said. "Please don't be scared to assist us at our time of need."
Thai authorities have strongly denied using the pair as scapegoats, insisting their case is built on solid evidence showing the DNA of the accused from initial tests matches samples taken from Witheridge's body.
The grisly murders delivered a fresh blow to the kingdom's image as a tourist haven after months of political protests that ended in May's army coup.
Martial law is still in place across the country, and tourist arrivals have eased off on last year.