Southwest Airlines Co said hundreds of its flights were delayed because of problems on its website, phone lines and airports on Sunday.
In a statement on its website, Southwest said intermittent technical issues "are impacting website performance in creating new bookings and requiring us to process some customers manually as they arrive for travel."
The airline, which flies in the United States, Mexico and the Caribbean, said it was asking customers to arrive at least two hours before their scheduled departures and use airport kiosks to print boarding passes and tags for baggage.
Southwest said that as of 3:30 ET, 450 of the approximately 3,600 flights scheduled were delayed. It did not specify the airports.
Southwest shares have risen for four consecutive months for a gain of more than 20 percent, but are still down 5.6 percent for the year. The yearly decline is an underperformance compared to the 2.1 percent decline in the S&P 500 over the same time frame.
(Reporting by Chuck Mikolajczak in New York; editing by Grant McCool)