(Reuters) -The U.S. Supreme Court rejected on Monday a request by American Airlines to overturn a judicial decision that found that the company's now-scrapped U.S. Northeast partnership with JetBlue Airways violated federal antitrust law.

The justices turned away an appeal by American Airlines of a lower court's decision in a lawsuit brought by the U.S. Justice Department that led to the end of the proposed "Northeast Alliance," which would have allowed the two carriers to coordinate flights and pool revenue.

The American Airlines argued that the antitrust violation ruling by the Boston-based 1st U.S. Circuit Court of Appeals wrongly embraced a hostility to collaboration between businesses and was contrary to the approach taken by other courts to require evidence of actual harm to consumers in the market as a whole, not just the customers of the collaborators.

The company also said the ruling invalidated a joint venture that increased market-wide competition among all airlines and "threatens to wreak havoc on productive collaborations of all shapes and sizes."

The November ruling came in a lawsuit the Justice Department filed in 2021 along with six states during Democratic President Joe Biden's administration. Under Biden, the Justice Department made boosting airline competition a top priority and aggressively enforced U.S. antitrust laws.

Despite a change in administrations, the Justice Department under Republican President Donald Trump has continued to defend the government's victory in the American Airlines-JetBlue case, saying the 1st Circuit's ruling upholding a judge's decision blocking the alliance rested on "uncontroversial antitrust principles."

The alliance was announced in July 2020 and approved by the U.S. Transportation Department just days before the end of Trump's first administration in January 2021.

Through their partnership, American, the nation's largest airline, and JetBlue, the sixth-largest, joined forces for flights in and out of New York City and Boston, coordinating schedules and pooling revenue.

The Justice Department argued that the alliance would hurt consumers by eliminating incentives for American to cut prices to lure customers from JetBlue, a historically disruptive rival with often lower fares.

U.S. District Judge Leo Sorokin in Boston in 2023 sided with the Justice Department and found the alliance violated antitrust law.

Following Sorokin's ruling, JetBlue terminated the alliance, as it unsuccessfully sought to bolster its efforts to win approval for the now-dropped $3.8-billion purchase of Spirit Airlines, which Biden's Justice Department also successfully challenged.

American Airlines, though, pressed ahead with an appeal, saying the ruling would prevent the company from entering into any similar future arrangement, including with JetBlue. But a three-judge 1st Circuit panel upheld Sorokin's decision.