Officials at London's Heathrow airport are warning of a second day of delays for travelers after a cyber attack disabled a passenger check-in and baggage system.

Passengers wait at Heathrow Airport in London, Britain, 21 in March 2025. The airport is dealing with a second day of flight delays after a cyber attack led to a shutdown of its passenger check-in and baggage handing system. File photo by EPA-EFE/TOLGA AKMEN UPI

Sept. 21 (UPI) -- Officials at London's Heathrow airport are warning of a second day of delays for travelers after a cyber attack disabled a passenger check-in and baggage system.

The problem resulted in hundreds of delayed flights at several European airports on Saturday and left workers using pen and paper to check passengers in for their flights.

At least 90% of the 350 scheduled departures at Heathrow had been delayed, most by at least 15 minutes. Six had been delayed as of Sunday afternoon, according to Flightradar24, which tracks air traffic. Thirteen flights were canceled on Saturday, but most of the hundreds of scheduled flights were delayed.

A Heathrow spokesperson said the "underlying problem was outside our influence" and added that the airport brought on additional staff to help manage the situation.

"We apologize to those who have faced delays, but by working together with airlines, the vast majority of flights have continued to operate," the spokesperson said.

There was no timeline for a return to a fully functional system, officials at Brussels Airport said, where authorities asked several air carriers to cancel at least half of their flights scheduled to depart on Monday.

RTX, owner of the software provider Collins Aerospace, said it was "aware of a cyber-related disruption" to its system in some airports, and that it was working to resolve the situation, the BBC reported.

The company has not disclosed the error in its software that caused the system to shut down, or how long the outage could last, but added that the disruption would stretch into at least until Monday because Collins has yet to provide a secure software update or any recommendations on a solution, airport officials said.

Airports urged travelers to check flight status before heading to the airport and recommended they arrive at their selected airport no earlier than three hours prior to long flights and at least two hours before shorter ones.