The same week multiple flights out of Denver International Airport were delayed due to a “gating issue,” Frontier Airlines is telling passengers to get ready for a scheduled system upgrade that could cause long lines at the ticket counter.

The upgrade started Thursday night and lasted into Friday morning. A 9NEWS photographer at the airport saw long lines at the ticket counter. Reporter Noel Brennan said security lines were around 30 minutes at DIA.

The upgrade is finished, and a Frontier spokesperson said things are running as normally as possible.

During the outage, check-in on Frontier’s website, the mobile app and airport kiosks wasn't available. The reservation system was also down, meaning passengers couldn't make changes like adding a bag anywhere other than the airport ticket counter.

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Because of this, Frontier asked anyone who needs to use the ticket counter to show up three hours early.

“We recognize that this may be inconvenient,” the airline said, “but it will help ensure we get everyone checked in and ready for departure.”

Frontier also wanted passengers to make sure they printed out their boarding passes before they arrive.

A spokesperson for DIA said the airport knows about the potential for hassles during the outage, and is on “standby mode to support if necessary.” Airport officials are monitoring the situation.