US Airways-American Airlines Merger Challenged
The US Justice Department and a number of state attorneys general have challenged the proposed $11bn (£7bn) merger between US Airways and American Airlines.
The Justice Department says the merger would result in the creation of the world's largest airline and reduce competition for commercial air travel in local markets.
A lawsuit filed in the federal court in Washington seeks to prevent the companies from making the deal in order to preserve head-to-head competition.
In a joint statement, the airlines said they would "mount a vigorous defence" of the planned merger, adding that blocking the deal would "deny customers access to a broader airline network that gives them more choices".
"We believe that the DOJ is wrong in its assessment of our merger," the statement read.
"Integrating the complementary networks of American and US Airways to benefit passengers is the motivation for bringing these airlines together."
"We will mount a vigorous defence and pursue all legal options in order to achieve this merger and deliver the benefits of the new American to our customers and communities as soon as possible."
Shares of both companies plunged on Tuesday, and other airline shares fell sharply as well.
In February, the two airlines disclosed their plans to create a company with 6,700 daily flights and annual revenue of roughly $40bn (£26bn).
But Attorney General Eric Holder said the transaction between US Airways and American would result in "higher airfares, higher fees and fewer choices".
Were the deal to be approved, the four biggest US airlines - American, United, Delta and Southwest - would all be the products of mergers that began in 2008.
Last year, business and leisure airline travellers spent more than $70bn (£45bn) on airfare for travel throughout the United States.
American parent AMR Corp has cut costs and debt since it filed for bankruptcy protection in late 2011.
Pilots from both airlines have agreed on steps that should make it easier to combine their groups under a single labour contract, a big hurdle in many airline mergers.
The attorneys general were from Arizona, Florida, the District of Columbia, Pennsylvania, Tennessee, Texas and Virginia.