"The suspects' detention has been extended five times despite there being no witnesses to their guilt. There are only the test results of DNA, semen and saliva claimed by police," he said.

"The suspects deny [the evidence] is connected to them,” he said. Mr Surapong said the arrest of the two men had become a huge issue, and the Myanmar embassy had taken up their cause in a way it had never done before.

Rights groups have expressed alarm after it emerged that lawyers representing Burmese migrant workers accused of killing two British tourists in Thailand have been given just a week to present their defence case, and without any access to the prosecution evidence.

The new timetable for the trial of the men accused of murdering Hannah Witheridge and David Miller on Koh Tao in September comes as more details were revealed about Thai police allegedly torturing other suspects and includes testimony from Burmese migrant workers on Koh Tao who say they were beaten, and scalded with boiling water.

Police eventually arrested Zaw Lin and Wai Phyo, both 21, a fortnight after the murders. The men were said to have initially confessed, but soon retracted this, claiming they too had been beaten and threatened with death by their interrogators.

Last week they were formally charged with beating the Britons to death with a garden hoe, and raping Witheridge, and face possible execution. According to Nakhon Chompuchat, the lawyer leading the defence case, the start of the trial has now been brought forward from 25 February to 26 December, with the court on Koh Samui, the larger island near Koh Tao, demanding the full defence case be lodged in the next week, by 18 December.

This must be done without the defence team being allowed to see the prosecution case, only a basic six-page document outlining the charges and some details of the injuries suffered by Witheridge, 23, from Norfolk and Miller, 24, from Jersey.

The British government, which sent a Metropolitan police team to Thailand to look over the inquiry, does have details of the prosecution case. These have been shared with the victims’ families, who last week said they considered the case against the Burmese men strong. However, none of it has been passed to defence lawyers.

Maya Foa from the fair trials group Reprieve, said the group extended deep sympathies to the victims’ families and “understand their desire to see that those responsible are held to account”.

She added: “We also share their desire to see a fair and transparent trial process. At the moment, however, this does not seem to be happening. There have been widely reported concerns about tainting of the crime scene, confessions extracted by torture and the unreliability of forensic testing. The defence team has not seen any of the prosecution evidence – despite it having been shared with the metropolitan police – and they now have been given only seven days to submit their case.

“The British government opposes the death penalty in all circumstances, yet it appears that their actions in this case could be contributing to death sentences for Burmese migrants after an extremely unfair trial. The UK should adhere to their policy on the death penalty and should urge the Thai authorities to conduct a fair and open trial that does not result in the executions of potentially innocent individuals.”

Andy Hall, a British activist who works with Burmese migrants in Thailand and has been assisting the defence team, said the lack of access to the prosecution case made the task of compiling the defence “almost impossible”.

He said: “Our team are confused about what evidence the prosecution have. We don’t know anything but media speculation. We’ll only know at the time we submit our own evidence. It’s an impossible situation – how do you prepare when you don’t know the case against you? That’s a principle of criminal justice, that you need to know the case against you to defend yourself.”

He added: “Also the UK government say they want a fair trial, but they’re also not sharing any information. They’ve got autopsy reports and witness statements of Britons who came back to the UK. If it’s such a strong case, why not share it?”

Separately, on Tuesday Hall passed to Thailand’s human rights commission a dossier of evidence he helped compile of allegations from other Burmese men living on Koh Tao who say they were seriously mistreated by police during the investigation into the killings. Thai police deny the claims.

Among the photographic, audio and video evidence were statements from young Burmese men who say they were beaten by local officers during questioning, and others who claim they were scalded with boiling water.

Other allegations include that suspects were questioned with their heads covered tightly with plastic bags, hands were behind their backs or they were beaten and threatened verbally.

Groups including Amnesty International have expressed worries at the investigation and the torture allegations. Among concerns are that DNA evidence allegedly linking the men to the crime could have been tainted after a large number of people trampled over the murder scene, and previous cases where Thai police have attempted to wrongly pin high profile crimes on migrant workers...

The truth is we are not really involved in this case.

We think the killer went to another country already.

We think it is injustice that we are in the jail.