''Human rights defenders are entitled to conduct peaceful human rights advocacy and to be protected from retaliation, including malicious prosecution and judicial harassment,'' says Lawyers' Rights Watch Canada in a strongly-worded letter.
The latest criminal defamation count, arising from an interview Mr Hall gave to the Aljazeera news outlet in Burma early last year, brings to four the number of charges brought by Thailand's Natural Fruit processing company against the 34-year-old Briton using criminal defamation, civil defamation and the controversial Computer Crimes Act.
As well as a lengthy jail term, Mr Hall also faces a claim for 300 million baht in compensation for Natural Fruit.
''The criminal charges against Mr Hall have been brought under defamation legislation that fails to measure up to international human rights standards,'' the Canadian lawyers tell British Foreign Secretary William Hague and Ambassador to Thailand, Mark Kent.
''Thailand's criminal defamation laws provide opportunities for malicious prosecution by persons and corporations wishing to silence critics and thereby continue possibly unlawful activities with no accountability.
''This view is affirmed by the recent US Trafficking in Persons report which states that use of Thailand's criminal defamation laws 'to prosecute individuals for researching or reporting on human trafficking may have discouraged efforts to combat trafficking.'''
Mr Hall's case reaches court as the coup commanders who took charge of Thailand on May 22 have undertaken to legitimise the employment of hundreds of thousands of Cambodians and Burmese in Thailand's fishing fleets and factories, and to make trafficking more transparent.
Generally speaking, British, Australian and European embassies stand back and allow the laws of Thailand and other developing countries to be used to prosecute their citizens, even when those laws fail to meet international standards.
Australia's Government and embassy has made no public protest over the unprecedented criminal defamation and Computer Crimes Act prosecution by the Royal Thai Navy of Phuketwan journalists Alan Morison and Chutima Sidasathian, even though the prosecution was criticised three times in the US TIP report.
The case is being brought over a paragraph republished in Phuketwan from a Reuters news agency special report on the trafficking of Rohingya boatpeople that formed part of a series which won the prestigious Pulitzer prize for investigative journalism. Reuters and the authors of the paragraph have not been charged.
In Mr Hall's case, according to the Canadian lawyers' rights group, ''the charges were filed as reprisals against Mr Hall for exercising his freedom of expression to expose violations by Natural Fruit of the rights of migrant workers employed in Thailand's tuna and fruit export industry.
''As such, the prosecution is illegitimate and contravenes Thailand's domestic and international legal obligations to ensure that all persons within its territory can exercise their lawful right to freedom of expression without risk of criminal sanctions.
''LRWC seeks the intervention of the UK government for protection of Mr Hall's internationally protected rights, including his rights pursuant to the International Covenant on Civil and Political Rights (ICCPR), ratified by UK in 1976 and by Thailand in 1996, and the UN Declaration on Human Rights Defenders adopted by consensus of the member States of the UN General Assembly in 1999.
''LRWC urges attendance and observation by UK officials at all proceedings, including a 'reconciliation' hearing on July 2 2014 at 8.30am.''
Mr Hall has said he does not intend to be intimidated into making admissions or an apology as part of any mediation.
The charges relate to a report entitled ''Cheap Has a High Price'' published in January 2013 by Finnwatch, an independent research organisation focused on global corporate responsibility issues. Mr Hall was a researcher on the project.
Thai authorities have so far failed to properly investigate the serious allegations of labor abuses made against Natural Fruit.
''The prosecution is an example of a corporation's shielding itself from accountability for wrongdoing by initiating criminal proceedings against a person reporting on alleged corporate wrongdoing,'' the Canadian lawyers' letter says.
''International concern with Thailand's inadequate response to reports of human trafficking and forced labor in several commercial sectors, including fruit manufacturing and fishing sectors, has resulted in Thailand being downgraded to the lowest possible level in the United States (US) State Department's 2014 Trafficking in Persons report (TIP report).''
Criminal charges are not matters ''between two private entities'' when an accused person is subject to restrictions on his liberty through bail requirements and a travel ban imposed through confiscation of his passport, and when the disposition of the case could result in lengthy jail sentences, the letter says.
''Both the criminal defamation law and the processes by which Mr Hall is being prosecuted fall conspicuously short of Thailand's international obligations. Thailand and the UK have a duty to ensure the protection of Mr. Hall as a human rights defender.''
British consular officials have reportedly advised Mr Hall that the embassy is ''unable to interfere in the judicial process.''
''This statement is at odds with international law,'' say the Canadian lawyers.
''Mr Hall has already been subject to violations of his fair trial rights through attempts to coerce a confession and guilty plea. Mr Hall has been charged with an offence that itself violates Thailand's legal obligations to protect the exercise of freedom of expression and other internationally protected rights for all persons within its territory.
''If convicted, Mr Hall faces lengthy arbitrary detention. This means there are no remedies available for Mr Hall in Thailand that comply with international human rights standards.''