(REUTERS) - A former American Airlines flight attendant trainee is due to be sentenced on Monday after admitting he made repeated bomb threats against a rival airline, mostly by calling 911 from pay phones, according to prosecutors in California.

The trainee, Patrick Cau, admitted to making eight separate bomb threats targeting United Airlines, starting last October and continuing for four months, according to a plea agreement reached with federal prosecutors last week. The agreement said the threats had caused significant disruptions and cost United US$267,912 (S$342,000).

In early October, for example, Cau called an internal United number to report a flight from London to Los Angeles would be bombed. In the other incidents, he made similar threats from pay phones in Los Angeles, New York, Las Vegas and Seattle, according to the plea agreement.

United had no immediate comment, and a lawyer for Cau did not immediately respond to a request for comment.

American Airlines said it had dismissed Cau in April, after his name came up on a no-fly list and that the airline had "followed all required security procedures."