The United States Government is to try to block a merger of American Airlines and US Airways.
The $US11 billion deal, which would form the world's largest airline, was backed by a federal judge in March and has been approved by the European Union.
The Justice Department said on Tuesday the merger would reduce competition and increase fares.
Six states and the District of Columbia have joined the action. The BBC reports they are Texas, where American Airlines is headquartered, Arizona, where US Airways is based, Florida, Pennsylvania, Tennessee and Virginia.
Shares in both companies fell, along with the stock prices of other airlines, after the announcement.
"By challenging this merger, the Department of Justice is saying that the American people deserve better," Attorney General Eric Holder said in a statement.
"This transaction would result ... in higher air fares, higher fees and fewer choices."
The lawsuit also cites direct competition between the airlines on nonstop routes worth about $US2 billion in annual revenues.