Wai Phyo and Zaw Lin, both from Myanmar, had denied killing David Miller and raping and killing Hannah Witheridge on the resort island of Koh Tao.
Their battered bodies were found on a beach on the morning of September 15, 2014.
The expert also alleged that police had failed to properly control the crime scene and mishandled the DNA evidence.
But a court rejected the defence arguments and in December 2015 convicted both defendants of murder and sentenced them to death.
Following Thursday's court hearing, Phil Robertson, Deputy Asia Director for Human Rights Watch (HRW) called on Thailand to "abolish the death penalty".
"It's a cruel and unusual punishment that violates international human rights standards," he added.
Following the initial ruling, HRW called the verdict “profoundly disturbing”, citing the defendants’ accusations of police torture that were never investigated and questionable DNA evidence linking them to the crime.
But Thailand’s Supreme Court has now upheld their convictions.
Miss Witheridge, 23, from Norfolk, and Mr Miller, 24, from Jersey, had arrived in Thailand separately and met at the hotel where they were both staying.
Their killers, who were both 22 at the time, were employed as service workers on the island, which is famous for is diving locations.