The US Justice Department, citing antitrust concerns, said it would block the merger of United Airlines and US Airways.
The department said the 16-month-old proposal, which would have created the nation's largest airline, would "reduce competition, raise fares and harm consumers".
Attorney-General John Ashcroft said millions of consumers would have to paymore for lower quality service if the acquisition were allowed to proceed.
Many analysts have viewed the $US4.3 billion merger as all but dead for months, saddled by rising costs and strong political opposition in Washington from critics who said it would have been bad for competition.
