Signatories to the letter include representatives of more than 20 countries, as well as global organisations including the International Trade Union Confederation, European Coalition for Corporate Justice (ECCJ) and Human Rights Watch.
The letter requests the removal of Mr Wirat Piyapornpaiboon, CEO of Natural Fruit as TIPA president and the revocation of Natural Fruit's membership in the association if it refuses to drop the case.
Natural Fruit filed its first criminal defamation charge against Hall in February 2013 after he contributed to a report by a Finnish NGO, Finnwatch, which reported serious rights abuses at its factory in Prachuap Khiri Khan province.
The report, 'Cheap Has a High Price,' utilised worker interviews to document rights violations including the use of child labor, violations of Thailand's minimum wage standards, the confiscation of migrant workers' travel and work documents, and the failure to provide legally mandated paid sick days, holidays and leave.
Hall, a British citizen, faces three criminal defamation charges, one civil defamation action, and two criminal charges under the Computer Crimes Act brought by Natural Fruit that could result in up to seven years in prison on each count and about $9.5 million in damages.
The trial is scheduled to begin in September 2014. The case against Hall has drawn widespread attention from the international labor and human rights community, as well as international media.
''Harassment of activists like Andy Hall, who stand up for the rights of workers, is an unacceptable assault on labor rights and freedom of speech,'' said Abby McGill, campaigns director for the International Labor Rights Forum.
''These charges, which were cited in Thailand's recent downgrade in the State Department's 2014 Trafficking in Persons (TIP) report, demonstrate how the Thai government and industry work together to silence criticism and cover up migrant worker exploitation, rather than deal with the systemic problems that allow it to continue.''
Frances O'Grady, General Secretary of the British Trades Union Congress, representing six million workers in Andy Hall's home country, said: ''Vulnerable workers need people like Andy Hall - working with trade unions in Thailand and internationally - to stand up against exploitation and abuse.
''We need to be free from harassment and victimisation so we can protect working people from corporate greed and government inaction. An attack on one of us is an attack on all of us.
''We are the workers, we are united. We will strongly defend for the rights of our migrant brothers and sisters and those who work for the workers' rights,'' said Komsan Tongsiri, General Secretary of State Enterprises Workers' Relations Confederation (SERC), Thailand.
The case comes at a time when Thailand has come under mounting pressure to address its labor and human rights situation, highlighted - on the international stage - by the US governments' Trafficking In Persons (TIP) report, which downgraded the country's rating to Tier 3, the lowest rating.
Declaration of Interest: Phuketwan journalists Alan Morison and Chutima Sidasathian face a continuing trial in March over criminal defamation and Computer Crimes Act charges brought by the Royal Thai Navy, citing a 41-word paragraph from a Pulitzer prize-winning Reuters report on the Rohingya boatpeople. The case was mentioned in June in the US State Department's Trafficking in Persons report, which downgraded Thailand to Tier 3. Reuters and several Thai mainstream outlets that carried the same paragraph have not been charged.