The ruling marked a victory for the Biden administration in its efforts to preserve competition among the lowest cost airlines to ensure that air travel remains affordable.
The judge wrote that the proposed merger "does violence to the core principle of antitrust law: to protect the United States' markets -- and its market participants -- from anticompetitive harm."
Spirit shares tumbled 60%, while Jet Blue shares fell 5% after the judge's decision.
The companies could still appeal the ruling.
JetBlue said it disagreed with the judge's ruling and is considering next steps.
The Justice Department did not immediately respond to a request for comment.
GARLAND ON MARCH 7, 2023:
"By acquiring Spirit, JetBlue will eliminate the largest ultra low cost carrier in the United States.
U.S. Attorney General Merrick Garland, along with six Democratic state attorneys general and the District of Columbia, had argued the deal would lead to fewer flights and higher prices for millions of Americans.
JetBlue's lawyers argued that the case was a "misguided" challenge to a merger between the nation's sixth and seventh largest airlines, which combined control less than 8% of a domestic market dominated by four larger carriers.
United, American, Delta, and Southwest Airlines control 80% of the market following a series of previous mergers that the federal government approved.
