(CNN) Thailand's Supreme Court has upheld the death sentence for two Burmese migrant workers convicted of murdering two British backpackers on a Thai resort island in 2014.
Zaw Lin and Wai Phyo -- also known as Win Zaw Htun -- were found guilty of the rape and murder of Hannah Witheridge, 23, and the murder of David Miller, 24, on December 24, 2015.
The bodies of the young Britons were discovered in September 2014 on a beach on Koh Tao, a small island in the Gulf of Thailand. They were partially undressed and had sustained severe head injuries.
The murders on Koh Tao -- an idyllic island popular with divers -- drew intense media attention from across the world. Defense lawyers for the two men later alleged that police had rushed the investigation in order to avoid Thailand losing its image as one of the world's most popular tourist destinations.
But Thailand's top court Thursday upheld the death penalty, ruling that forensic evidence against the convicted men was "clear, credible, and detailed." The court also refuted suggestions that the police had mishandled the case.
Speaking outside the courtroom in Bangkok, head defense lawyer for the pair, Nakhon Chomphuchat, said they would seek a royal pardon, which must be submitted within 60 days.
The Burmese men, from Myanmar's Rakhine state, were working in the hospitality industry on the island at the time of the incident. They were arrested almost two weeks after the bodies were found and originally confessed, only to later recant, saying their admissions of guilt were made under duress.
The defense team argued that the police investigation was flawed due to "alleged mishandling of forensic evidence, abuse of suspects and intimidation of witnesses," according to a previous statement issued by the Migrant Worker Rights Network, a rights group assisting the defense team.
Thai police said forensic evidence, including DNA samples from cigarette butts found near the bodies, tied the men to the scene.
In 2015, the two men -- both 22 -- were found guilty of murder and sentenced to death. An appeals court upheld the guilty verdict in 2017.
Andy Hall, international affairs adviser to the official legal defense team, said that while he respected the decision of the court, the death penalty sentence against the two should "be reversed and quashed."
He added that the forensics evidence relied upon to convict the pair 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.
In June last year Thailand lifted a de facto moratorium on the use of the death penalty, executing a man by lethal injection in the country's first execution since August 2009, rights groups said.