Lawyers representing Burmese migrant workers accused of killing two young British tourists in Thailand have accused the Foreign Office of being complicit in ensuring the men will not receive a fair trial after officials in London refused to share any information about the prosecution case.

The legal team representing Zaw Lin and Wai Phyo, who will stand trial later this month for the murders of Hannah Witheridge and David Miller on the holiday island of Koh Tao in September, say information held by the Foreign Office is the only way to access the prosecution case ahead of the court case. The defence lawyers must submit their case within the next 10 days, but are not being allowed by the Thai court to see any of the prosecution evidence in advance.

The Foreign Office had “not been helpful or proactive in any way”, said Nakhon Chomphuchat, the human rights lawyer who is leading the defence case. “The defendants cannot fairly fight the case against them until their lawyers are able to know the case against them. The prosecution has not even provided witness lists, so it’s impossible to plan the defence strategy.”

The issue is a complex one given the British government has no formal legal duties over the case, beyond representing the interests of the families of Witheridge, 23, from Norfolk and Miller, 24, from Jersey. However, when the Burmese men retracted initial confessions soon after their arrest, saying they had been tortured by their interrogators, Britain expressed public worries about the investigation, even calling in a senior Thai diplomat to express concern. Later, Metropolitan police detectives were sent to Thailand to look over the Thai case, and report back to officials and the families.

In the wake of the Met police report, which has not been made public, the Foreign Office released statements from the Witheridge and Miller families in which they expressed confidence in the Thai investigation and criticised the media for widespread reports into concerns Zaw Lin and Wai Phyo might have been used as scapegoats. The Millers’ statement said: “From what we have seen, the suspects have a difficult case to answer. The evidence against them appears to be powerful and convincing. They must respond to these charges, and their arguments must be considered with the same scrutiny as those of the prosecution.”

The defence team argue that while they cannot speculate on the suspects’ guilt or innocence it is imperative they receive a fair trial, especially given previous cases in Thailand where Burmese workers have been wrongly accused of high-profile crimes.

The lawyers have repeatedly asked the Foreign Office if it can share any information to help them put together a defence case. On Monday an FCO official replied, saying this was not possible.

The email, from an official in the South-East Asia department, passed to the Guardian by Chomphuchat, says they could only pass on information with the permission of Thai authorities or the trial judge.

The Met police team in Thailand had only reviewed the investigation “to provide reassurance” to the victims’ families, the FCO email explained, meaning the primary evidence “remains in the possession of the Thai police and prosecutor”. The only possible information could come from the Jersey and Norfolk coroners, who hold copies of UK autopsy reports on Witheridge and Miller, it added, saying it is up to the coroners to decide whether they can share this.

Chomphuchat said the defence team was frustrated that the FCO should release statements talking about a “powerful and convincing case” and then say they cannot share the information that prompted the families to come to this view.

“It is the families’ right to be kept informed like this, but not that the FCO facilitated the public release of this information to the mass media,” he said. “This has undermined the fairness of the trial, and possibly prejudiced the trial.I think the UK government is not helping as it doesn’t want to help. Perhaps they don’t want any problems, or accusations they might be assisting person who turn out to be guilty.”

The Foreign Office has previously said it released the family statements in the same way it would in any other similar case, and that this did not mean it necessarily shared the views.

Chomphuchat and his team have been told by the court in Koh Samui, the larger island near Koh Tao, where the men will be tried, that they must submit their full list of witnesses by Thursday, ahead of a trial starting on 26 December. Another worry, he said, was that the prison on Koh Samui was refusing to arrange a room for him to meet the suspects to discuss the case. Instead, he has had to speak to them one at a time via telephone behind a plastic screen.

Witheridge was raped and beaten to death on a beach in the early hours of 15 September, while Miller was beaten about the head and left to drown in shallow surf. Zaw Lin and Wai Phyo were arrested a fortnight after the murders, amid a frantic police hunt which had already seen Thailand’s military prime minister, General Prayut Chan-O-Cha, say he believed Burmese migrant workers were most likely to blame. As well as their torture claims, other Burmese migrants on Koh Tao have told human rights groups they were beaten and scalded by police seeking information.

A government spokesperson told the Guardian:

“The evidence to be presented to the court was and remains in the possession of the Thai police and prosecutor. Decisions about what and how this will be presented at any trial are for the Thai authorities to make. The British Government cannot interfere in Thailand’s judicial proceedings, just as other governments are unable to interfere in our own judicial processes. We have called for the investigation to be conducted in a fair and transparent way, in line with international standards.”