The families of two migrant workers sentenced to death for murdering backpacker Hannah Witheridge have appealed to the Thai king to spare their lives.

Great Yarmouth Mercury: Wei Phyo wearing the white shirt and Zaw Lin wearing the red shirt. Photo taken inside holding cell prior to incarceration in Koh Samui Provincial prison. Picture: Supplied.Wei Phyo wearing the white shirt and Zaw Lin wearing the red shirt. Photo taken inside holding cell prior to incarceration in Koh Samui Provincial prison. Picture: Supplied. (Image: Supplied)

The mothers of Burmese men Zaw Lin and Wai Phyo, accompanied by lawyers and a senior diplomat from Burma's embassy in Thailand, submitted an official petition for clemency on Thursday (October 24) at Bang Kwang prison on the outskirts of Bangkok, where the two men are being held.

The pair had denied killing and raping Miss Witheridge, from Hemsby, who died aged 23 while backpacking on the Thai island of Koh Tao in 2014.

They had also denied killing David Miller.

The pair's bodies were found on the beach on the morning of September 15, 2014.

Great Yarmouth Mercury: Phyu Shwe Nu, mother of Myanmar Zaw Lin, center left, and May Thein, mother of Myanmar Wai Phyo, center right, arrive for petition royal pardon at Bangkwang prison on the outskirts of Bangkok, Thailand, Thursday, Oct. 24, 2019. The families of two Myanmar migrant workers sentenced to death for the brutal murders of two British tourists in Thailand in 2014 have appealed to the Thai king to spare their lives. (AP Photo/Sakchai Lalit)Phyu Shwe Nu, mother of Myanmar Zaw Lin, center left, and May Thein, mother of Myanmar Wai Phyo, center right, arrive for petition royal pardon at Bangkwang prison on the outskirts of Bangkok, Thailand, Thursday, Oct. 24, 2019. The families of two Myanmar migrant workers sentenced to death for the brutal murders of two British tourists in Thailand in 2014 have appealed to the Thai king to spare their lives. (AP Photo/Sakchai Lalit)

The verdicts were controversial because of allegations that police mishandled evidence and beat the suspects into making confessions.

Lawyers for the two men had claimed evidence used in the case against them was mishandled and they made confessions under duress that they later retracted, raising questions about police competence and the judicial system in Thailand.

Human Rights Watch called the verdicts "profoundly disturbing", citing the defendants' accusations of police torture that were never investigated and questionable DNA evidence linking them to the crime.

Dressed in traditional clothing, the two mothers - May Thein and Phyu Shwe Nu - said they hoped the king would grant their plea and reduce the punishment to life imprisonment.

"We believe our sons are innocent," said May Thein, mother of Wai Phyo. "Many people believe the same thing as us."

Miss Witheridge and Mr Miller, 24 and 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.

During the trial, a well-known Thai forensics expert testified that the DNA evidence, which formed a major element of the prosecution's case, did not link the defendants to the scene.

The expert also alleged that police had failed to properly control the crime scene and mishandled the DNA evidence.