The airline's stocks fell 75 percent from $12.30 to $3 per share, the Chicago Tribune reported.
The outdated story, which concerned United filing for bankruptcy, had appeared five years ago in the Chicago Tribune and again this morning on the South Florida Sun-Sentinel Web site. It was later picked up by the Bloomberg financial news service.
Sun-Sentinel Deputy Managing Editor-Interactive Joe Schwerdt said records showed no one at the Sun-Sentinel had opened the story, which was available on their site since 2003. Further, no one outside the paper should have had access, Schwerdt said.
United issued a statement saying it would investigate the incident. It also demanded a retraction of the story.
"United continues to execute its previously announced business plan," the airline's statement said.
The airline emerged from bankruptcy in February 2006, the Tribune reported.
As word got around the story was old, share values began to recover, most recently trading at $8.87 per share, the Tribune said.
The South Florida is a Tribune company-owned newspaper.