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RIO DE JANEIRO (Reuters) - Brazilian investors’ association Abradin said it would file a lawsuit claiming a proposed tie-up between U.S. planemaker Boeing Co BA.N and Embraer SA EMBR3.SA should come with a public tender offer, escalating its fight against the proposed deal.

The group had already filed a motion last month with a São Paulo court questioning the legality of the deal and seeking to convince that court that Embraer shareholders should be eligible for a public tender offer.

In July, Embraer agreed to sell 80 percent of its commercial plane division to Boeing for $4.2 billion, as global competition escalated between the U.S. planemaker and Airbus. The deal has been approved by Embraer’s board and the Brazilian government but has yet to be put to a vote by its shareholders.

Embraer’s bylaws ensure an offer to all shareholders with a 50 percent premium over market prices if an investor buys 35 percent or more of the company.

If the deal goes through, Embraer will have to attempt to be profitable from its executive and defense divisions which have posted losses in recent years, as well as from dividends it will receive from its remaining stake in its commercial division.

“What will be left behind with Embraer will not survive in the medium term without heavy government subsidies,” said Aurelio Valporto, president of Abradin.

Embraer declined to comment on the matter.

The Brazilian planemaker said this month it expected to post little to no profit over the next two years.

The proposed Boeing-Embraer deal faces challenges on multiple fronts, including from left-wing politicians and labor groups who say the tie-up would lead to job cuts. The deal has been temporarily blocked by Brazilian judges at least twice through injunctions, both of which were swiftly reversed.