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 pay more 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.
United Airlines-US Airways merger blocked
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