The decision, opposed by U.S. airlines and labor unions, could increase competition on trans-Atlantic routes.

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DALLAS — Norwegian Air Shuttle has won preliminary approval to expand flying between the United States and Europe using an Irish subsidiary.

The decision Friday by the U.S. Department of Transportation was a setback for the major U.S. airlines and their labor unions, who said the subsidiary will be an unfair, low-wage competitor on key transatlantic routes.

The decision followed intense lobbying and public-relations campaigns by both sides and an unusually long review — more than two years — by regulators.

The Transportation Department said that it will take public comments before issuing a final order that could allow the Norwegian subsidiary to begin flying to the U.S.

Norwegian, a low-fare, no-frills airline, called the decision a victory for consumers and said it intended to hire U.S.-based crews.

Bjorn Kjos, CEO of Norwegian Group, said final approval for the U.S. flights “will be win-win for consumers and the economy on both sides of the Atlantic.” He said Norwegian would create “thousands” of jobs and help the economies of its U.S. destinations.

U.S. airline unions immediately criticized the decision. Sara Nelson, president of the Association of Flight Attendants, said the Obama administration “has chosen foreign corporations over workers’ rights and good American jobs.”

American carriers and their unions had argued that Norwegian decided to expand U.S. service using an Irish subsidiary because Ireland’s tax and labor laws are weaker than those in the United States. They said Norwegian would gain an unfair advantage and undercut the American carriers on fares.

American Airlines, Delta Air Lines and United Airlines all declined to comment Friday. All three operate flights across the Atlantic and have European partners that do the same.

Norwegian, a Boeing customer that flies 737s on its European routes, also has 19 787-9 jets on order. It intends to use 787s to fly from the U.S. to European cities and from there to Thailand and other Asian destinations

The Norwegian subsidiary initially plans to fly between Boston and Cork, Ireland.