UPDATE 2-Computer failure grounds United flights

(Recasts, adds flights canceled, updates stock price)

CHICAGO, June 20 (Reuters) - A two-hour computer outage affecting dispatch operations at United Airlines, a unit of UAL Corp. (UAUA.O), delayed nearly 270 domestic and international flights by the No. 2 U.S. carrier on Wednesday.

Twenty-four domestic flights were also canceled as a result of the systemwide failure, the airline said in a statement. United had not determined what caused the problem.

"We continue to work hard to resume operations by tomorrow morning and kindly ask for our customers' patience," the airline said.

At 8 a.m. Central time (9 a.m. EDT), the outage struck a computer program that measures the weight of aircraft before departure. The information is crucial for operational and safety purposes.

Delays averaged about 90 minutes and no other airline was affected by the outage.

It is uncommon for serious delays to result from airline computer problems. A major failure in a U.S. Federal Aviation Administration computer combined with severe thunderstorms in the eastern United States caused serious delays on June 8.

Service problems at U.S. airlines have received more scrutiny in recent months, particularly the industry's on-time performance.

UAL shares rose 74 cents, or 1.95 percent, to close at $38.67 on Nasdaq.