The mothers of two migrants sentenced to death for the murder of two British backpackers in Thailand have submitted a plea for clemency.

Zaw Lin and Wai Phyo were sentenced to death in December 2015 for the murder of Hannah Witheridge, 23, and the killing of David Miller, 24, who were found bludgeoned to death on a beach on the island of Koh Tao in September 2014.

Lin and Phyo initially confessed to raping Miss Witheridge before beating her and Mr Miller to death, but later recanted, saying they were tortured.

Following the murders, Koh Tao earned the nicknamed of 'death island' due to the number of western tourists who have died or gone missing there.

Now their mothers have submitted a plea for clemency from the Thai king Thursday, in a case tainted by claims of irregularities - saying the confessions by the pair were obtained under duress.

Phyu Shwe Nu's son Zaw Lin is currently on death row in Thailand for the alleged murder of two young British tourists (she is pictured holding a pictured of her son earlier this month)

Myanmar migrant workers Zaw Lin (left) and Win Zaw Tun (right) were sentenced to death by Thai courts (pictured in August)

Hannah (left), 23, and fellow traveller David Miller (right), 24, were murdered on a beach in Thailand in 2014

Defence lawyers have also said the evidence used to convict the two men was unreliable as authorities had mishandled DNA and did not allow independent analysis of the samples.

Defence appeals were exhausted in August and a royal pardon or commutation of the death sentence by the Thai king is the last chance of a reprieve.

'We are not challenging the judgement. We are saying that by giving them the death penalty, they will lose the opportunity to do something good with their lives,' lawyer Nadthasiri Bergman said.

She joined the mothers of the two men, and Myanmar embassy officials, to submit the request at the sprawling Bangkok prison where they are being held.

'They deserve a second chance,' she added.

In the request Zaw Lin and Wai Phyo have pleaded for 'compassion from the king,' Andy Hall, advisor to the legal defence team, added.

The original verdicts on the 2014 double killing divided relatives of the victims (pictured, Phyu Shwe Nu, the mother of Zaw Lin, hangs pictures of her son on the wall earlier this month)

Phyu Shwe Nu, the mother of Zaw Lin, looks at photos of her son on the wall of her home in Michaung Yaythauk, a village only accessible by boat in Myanmar's western Rakhine state

A general view of the house of of Zaw Lin, who is currently on death row in Thailand for the alleged murder of two young British tourists. His home is located in the village of Michaung Yaythauk, which is only accessible by boat in Myanmar's western Rakhine state

Defence lawyers have said the evidence used to convict the two men were unreliable as authorities had mishandled DNA and did not allow independent analysis of the samples (pictured, Maung Zaw Win, the older brother of Zaw Lin)

The original verdicts on the 2014 double killing divided relatives of the victims.

Miller's parents backed the court's decision, but the family of Witheridge were more cautious, with her sister saying she believed the investigation had been 'bungled'.

The case has triggered a strong reaction from Myanmar, where many felt the two men had been given an unfair trial as low-paid migrant workers -- an integral part of Thailand's workforce -- are often regarded with contempt by its public.

The police were accused of buckling under pressure to solve a crime that attracted global attention.

Zaw Lin's brother who was working alongside him in Thailand at the time of the murder said earlier this month that the two were simply used as 'scapegoats' to save Thailand's image.

David and Hannah were found bludgeoned to death on this Thai beach early in September 2014, shortly before Lin and Phyo were arrested (pictured, tributes at the scene)

Lin (left) and Phyo (right, both wearing helmets) initially confessed to the killings and were taken to the murder scene to reconstruct what they said happened, but later recanted

'Hearing the judge give him (Zaw Lin) the death sentence was horrific,' Maung Zaw Win told AFP at their family home in western Rakhine State -- one of Myanmar's most impoverished.

'I ask the Thai King to show kindness to them, to show them humanity,' he added.

It is unclear when the sentence will be carried out, with hundreds remaining on death row.

But last year Thailand carried out its first execution since 2009, prompting criticism by rights groups who had hoped the country was moving towards abolishing the practice.