Washington ( CNN ) -- The Justice Department and attorneys general from six states and the District of Columbia filed a lawsuit Tuesday challenging the $ 11 billion merger of American Airlines and US Airways , saying the combination would lead to higher prices and less service for consumers . The merger , which would create the world 's largest airline , would `` substantially lessen competition '' for commercial air travel , contends the complaint , filed in U.S. District Court in Washington . The carriers said in a joint statement that they would mount a `` vigorous and strong defense '' of their deal and pursue all legal options to move it forward . `` We believe that the DOJ is wrong in its assessment of our merger . Integrating the complementary networks of American and US Airways to benefit passengers is the motivation for bringing these airlines together . Blocking this procompetitive merger will deny customers access to a broader airline network that gives them more choices , '' the companies said . Opinion : Why American-US Airways deal is good . US Airways shares slid more than 9 percent on Wall Street . American is operating under Chapter 11 bankruptcy protection . A top concern is local markets , the Justice Department said . It cited the example of Ronald Reagan Washington National Airport , where the combined airline would control 69 % of takeoff and landing slots , and 63 % of the outbound nonstop routes . The Justice Department also said in its complaint that if the two airlines were allowed to merge , there would be higher fees for baggage and flight changes . The states that joined the antitrust lawsuit are : Texas , where American 's parent company AMR Corp. is based ; Arizona , home base for US Airways ; Florida ; Pennsylvania ; Tennessee ; and Virginia . The lawsuit comes as somewhat of a surprise . In recent years the Justice Department has allowed other big airline mergers -- Delta gobbled up Northwest , and United absorbed Continental -- in part because the airline industry was losing so much money and the mergers were seen as necessary for survival . But airlines are now making billions of dollars , not only from higher fares but also from $ 6 billion in fees they collected in 2012 alone , for baggage and seat selection and other things that used to be included in fares . Minority businesses would fight any US Air merger . American and US Airways had touted their combination as one that `` joins two highly complementary networks with access to the best destinations around the globe -- offering you increased choices , improved efficiency and better options '' under American 's Oneworld alliance . The Justice Department lawsuit cites internal documents from the airlines to show that the merger is n't needed and in fact would lead to cuts in programs that benefit travelers . An example is US Airways ' Advantage Fares discount program , which executives said would likely be eliminated after the merger , according to the Justice Department . Without the merger , antitrust officials said , other airlines would be forced to continue to match US Airways ' discounts to compete . American has been in bankruptcy protection since 2011 and has been working to get approval from antitrust authorities and shareholders for the merger so it can emerge from court protection . The companies had hoped to complete the deal by the end of September . The European Union this month cleared the merger with the condition that the airline give up slots to allow new competition in the Philadelphia-to-London route . CNNMoney 's Chris Isidore contributed to this report .
The merged American Airlines - US Airways would be the world 's largest airline . Lawsuit : The merger would ` ` substantially lessen competition '' for air travel . Previous mergers won approval in recent years amid the poor economy , but it now loses money . NEW : American / US Airways say they will fight government attempt to block their deal .