Petrobras faces a multibillion-dollar tax bill over the oil giant’s chartering of vessels to foreign companies after an administrative panel rejected an appeal.

The Rio de Janeiro-based company said that a panel of the Superior Chamber of Tax Appeals issued an “unfavourable ruling” that could cost it BRL 18bn ($3.4bn).

Petrobras said the ruling involves import duties on charter deals for vessels from international markets in 2010, 2011 and 2013.

The government-controlled company had filed its appeal with the tribunal to determine whether it owed duties known as Brazil’s Contributions for Intervention in the Economic Domain, which cover fuel imports.

Also at issue were federal taxes to Brazil’s unemployment insurance and social security systems.

But a majority of the panel’s members rejected the Petrobras appeal.

The appeals chamber is the final administrative step for the oil and gas company in its efforts to avoid the costly tax bill, so further challenges may have to take place in Brazil’s federal court system.

“The company, after the outcome of the administrative process, will adopt the appropriate legal measures to challenge the collection, as well as guarantee the debt,” Petrobras said in a filing to the US Securities & Exchange Commission.

Sao Paulo and New York-listed Petrobras said losing the battle remains possible, and the latest administrative decision is not yet factored into its financials.

The full text of the tax appeal ruling was not immediately available to the public.