A Boeing 737-800 from low-cost airline Norwegian Air Shuttle flies near Oslo airport in Gardermoen, Norway. A subsidiary of the company has been approved by the U.S. to fly out of a base in Ireland. (Kyrre Lien / AFP/Getty Images)

Despite opposition from American carriers, the U.S. Department of Transportation issued a final permit Friday to allow Norwegian Air International to fly in and out of the U.S. from a base in Ireland.

Opponents, including major U.S. carriers and airline labor groups, say Norwegian Air International is a subsidiary of a carrier based in Norway, but was created and based in Ireland to circumvent Norway’s more strict labor and tax laws and undercut the fares charged by rivals flying between Europe and the U.S.

But in approving a final ruling on a flight permit, the Department of Transportation said it found no legal reason to block the carrier from making regular flights into the country.

“Regardless of our appreciation of the public policy arguments raised by opponents, we have been advised that the law and our bilateral obligations leave us no avenue to reject this application,” the agency wrote in its final order.

Norwegian Air International is a subsidiary of Norwegian Air Shuttle, one of Europe’s largest low-cost carriers. Norwegian Air Shuttle already flies into the U.S., but by creating a new Irish subsidiary, the parent company said it can more easily compete on flights between Europe and the U.S.

Norwegian Air officials have defended their application, saying they plan to abide by all tax, labor and safety regulations in the countries they serve.

But opponents say they are not giving up the fight yet.

Edward Wytkind, president of the Transportation Trades Department, an umbrella group for transportation labor unions, said his group plans to appeal the decision to President Barack Obama and President-elect Donald Trump, if needed.

He said his group worries that other foreign airlines may take similar action to create subsidiary airlines in countries with low taxes and hire foreign pilots to slash fares.

“Competition is fine as long as the competition is fair and everyone plays by the same rules,” Wytkind said.

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