Zaw Lin and Win Zaw Tun were found guilty of killing David Miller, 24, and the rape and murder of Hannah Witheridge, 23, whose battered bodies were found on a beach on the southern diving resort of Koh Tao in September 2014.
Prosecutors insisted the evidence against the migrant workers was rock solid.
But during the trial and in their appeal, the defence said police bungled their investigation from the outset after local detectives came under huge pressure to solve a crime that risked damaging the country's vital tourism sector.
Investigators were accused of failing to properly collect and preserve DNA samples and declining to test key pieces of evidence, or allow independent examination of the samples.
Their appeal failed on February 23, but was only made public on Wednesday, even catching the defendants' lawyers flat-footed.
‘The Appeals Court upheld the Criminal Court's ruling against the two Myanmar suspects,’ Theerawut Phamhun, deputy provincial prosecutor of Surat Thani province, told AFP.
He said the appeal judges ruled the evidence investigation ‘was up to standard’ and that DNA found at the scene matched the suspects.
Nakhon Chomphuchat, the lead lawyer for the two accused, told AFP his team had only been made aware of the court's ruling on Wednesday.
‘I have yet to see my clients,’ Nakhon added.
It means the two accused had no legal representation at the ruling.
Defence lawyers now have 30 days to appeal to Thailand's Supreme Court, the final court, though it is unclear when the timer for that deadline started.
Thailand's legal system is notoriously opaque, with some cases taking years to heard while others are hustled through the courts.
It is often hard for press to access judgements or hearings.
While Thailand keeps the death penalty on its books, it is rarely carried out.
The double killings caused revulsion.
Miller was struck by a single blow and left to drown in shallow surf while Witheridge was raped and then bludgeoned to death with a garden hoe.
Her injuries were particularly horrific.
The parents of Miller backed the court's conviction and the police investigation.
Witheridge's family were more cautious in drawing conclusions while her sister Laura later wrote a scathing critique of the Thai police on Facebook calling the investigation ‘bungled’.
