Britain’s richest man, Sir Jim Ratcliffe, and his chemicals company Ineos are facing a backlash over plans to build two chemical plants in Antwerp after a Flanders transparency watchdog ruled that key documents may have to be disclosed.

The Brexit-backing billionaire announced in January that Ineos would invest €3bn (£2.6bn) in two plants at the port of Antwerp. Ratcliffe, the founder and chief executive of the petrochemicals company, said the investment was “the largest of its kind in Europe in more than a generation”.

But campaigners have questioned how the plants meet Belgium’s climate change commitments and ambitions to cut waste. “Every single angle – whether it is plastics, or the economic angle, or climate change – every single one, I have come to the conclusion that this factory is not good for Flanders or for Europe,” said Rob Buurman, director of the Recycling Network, which campaigns to reduce plastic waste. “We don’t really need this factory; for me the solution is not to start making more or cheaper plastics.”

The port of Antwerp, as well as the Flanders Investment & Trade agency, which played a crucial role in bringing the business to Belgium, turned down Buurman’s request to release documents about the project.

Now an official transparency watchdog has ruled the two organisations must release documents, unless there is a good legal reason not to. While some documents are likely to remain classified to protect commercial secrets, those familiar with the case believe many will have to be released following the decision by the appeal body of the Flemish government for public administration.

The disclosure could reveal how Antwerp lured Ineos to its port, beating a rival bid from the Dutch port of Rotterdam.

The decision to build two chemicals plants at Ineos’s existing site at the port of Antwerp was applauded by local politicians. The region’s minister-president Geert Bourgeois said it was “the largest investment in Flanders in the past 20 years”.

The plants will convert propane into chemicals that can be used in parts for cars, construction, cosmetics, pharmaceuticals, electronics and packaging. It is scheduled to be up and running by 2024, and Ineos said it would create 400 jobs, as well as 3,000 indirect jobs during the construction phase.

Sara Van Dyck of Flemish NGO Bond Beter Leefmilieu said she welcomed the decision to order the release of documents about the project. “We are curious to know what the government has promised the company to invest there and we hope no disproportionate advantages have been promised.”

She said her organisation, which campaigns on environmental issues, was not opposed to the project but had “many many concerns”.

The NGO wants Ineos to show how its plants would ensure zero carbon emissions and complete recycling of raw materials by 2050. “We know we are heading towards climate neutrality because [Belgium] signed the Paris agreement,” she said. “The longer we don’t act, the bigger the risk will be that there won’t be the necessary [green] investments and the CO2 crisis will get worse and worse.”

A spokesperson at the Antwerp port authority said “a large number of documents” relating to Ineos’s investment had been shared but the organisation believed it was not possible to release others because of commercial interests related to continuing negotiations.

“We have taken note of the decision of the appeal body of the Flemish government for public administration regarding the Ineos file and we are currently analysing it,” the spokesperson said. “In execution of the decision, we’ll return to the appeal body by 9 May, the official deadline.”

Claire Tillekaerts, the chief executive of Flanders Investment & Trade, said it was investigating the decision of the appeal committee (beroepsinstantie), in consultation with its counsel.

Ineos did not immediately respond to requests for comment.