Intervention by the union group has the potential to escalate the Thai dispute to international levels and trigger boycotts or importation restrictions on Thai products.
In a strongly worded letter to Prime Minister Yingluck Shinawatra, ITUC General Secretary Sharan Burrow urges Thai authorities to ''drop the two criminal charges against Andy Hall, as they incriminate legitimate and peaceful himan rights work.''
The letter lifts the dispute to one of national importance in Thailand and will draw the attention of world's largest retailers to the Thai brands involved.
The intervention in the case by the ITUC, which represents more than 174 million workers in 156 countries including Thailand, will also spark broader international media coverage of the attempt to silence and punish Mr Hall.
The 33-year-old labor campaigner says any court appearance he is forced to make will spotlight Natural Fruit, the company taking proceedings against him, and other Thai food exporting companies.
Mr Hall has become prominent in defending the rights of Burmese and Cambodian workers to proper wages and conditions in all industries throughout Thailand.
The Natural Fruit Company has taken action against Mr Hall that could see him jailed for up to two years and fined 300 million baht. The trial could begin as soon as next Wednesday.
The ITUC letter to PM Yingluck concludes: ''We hereby express our serious concern with the lawsuit filed against Andy Hall as the charges are based solely on legitimate human rights work . . .
''These legal actions are an attempt to silence human rights defenders in Thailand who, peacefully and legitimately, exercise their right to freedom of opinion and expression, in order to defend and promote human rights.
''We therefore urge the Thai authorities to:
''1. Drop the two criminal charges against Andy Hall, as they incriminate legitimate and peaceful human rights work;
''2. Ensure that the right to freedom of opinion and expression, as guaranteed under international law and as stated, inter alia, in Article 19 of the International Covenant on Civil and Political Rights, to which Thailand is a state party since october 29, 1996, is duly respected;
''3. Guarantee in all circumstances that all human rights defenders in Thailand are able to carry out legitimate human rights activities without fear of reprisals and free from any restrictions, including judicial harassment.''