The planned purchase would reduce competition, raise prices for travelers, and lower the quality of service, officials said.
"The proposed acquisition would allow Northwest to acquire voting control over Continental, as well as share in Continental's profits, diminishing substantially both Northwest's and Continental's incentives to compete against each other," the department said in a written statement.
A complaint filed in U.S. District Court in Detroit said Northwest and Continental are each other's most significant competitors for nonstop services between hubs at Detroit; Memphis, Tenn.; Minneapolis, Cleveland, Houston, and Newark, N.J.
"The two airlines also have a dominant share of the traffic on connecting flights between numerous cities," the department said. "Millions of passengers spend more than $350 million each year traveling between these cities."
Northwest is headquartered in Eagan, Minn., and Continental is based in Houston.
Under the deal, announced in January, the airlines would keep separate names, fleets, and employees, but the Justice Department is treating the alliance as a merger.
The announcement spurred the rest of the nation's six biggest carriers to hurriedly put together partnerships of their own, leading to fears that the consolidation could reduce service and drive up fares.
Written by Cassandra Burrell