Finnwatch's research, conducted from October to December, found serious labour rights abuses at the facilities of Natural Fruit Company in Prachaub Kiri Khan province, 230 kilometres south-west of Bangkok.
The company is a key supplier to Dutch-based Refresco Group BV, which reportedly controls 20 per cent of the European private label soft drink market.
"It is unusual that a company producing directly to the international market and with many high-profile customers is involved in such gross violations of fundamental rights," said Finnwatch researcher Henri Purje.
Refresco's clients include Lidl, Aldi, Carrefour, Dia, Morrison, Edeka, Rewe, Superunie, Ahold and Systeme Uni and well as Finland's largest chains Kesko, SOK and Suomen Lahikauppa, Finnwatch said.
Its investigation of Natural Fruit found there were 200 undocumented migrants working in the factory, some of whom were 14 years old. The minimum working age in Thailand is 18.//DPA