(Bloomberg) -- A major air-traffic control outage last summer in the UK was exacerbated by poor communication choices and a slow response to the incident, an interim report found.

According to the report released by the UK Civil Aviation Authority on Thursday, airlines and airports said warnings about the software issue were delayed during the Aug. 28 mishap, leading to additional disruption for passengers. There also wasn’t enough engagement from senior executives at UK air traffic control service NATS after the event. For example, NATS chose to communicate the issue through an “old-fashioned teleconference platform” instead of a video call, the report said.

The report lays out in damning detail how a mix of poor planning, slow action and lack of expertise disrupted travel for more than 700,000 passengers and cost airlines like British Airways millions to reimburse customers. A previous report found the outage was caused by an anomaly in NATS’ software when processing an airline’s flight plan. The system as well as its secondary backup then shut down for safety and took hours to reboot.

When the software glitch occurred, experienced engineers weren’t on site to restart the system, and it took an expert 90 minutes to reach NATS headquarters in Swanwick because they were working from home, the report said. It also took three hours to identify the right personnel to help fix the issue, though the engineer then turned out to be unfamiliar with the system’s fault message, further prolonging the failure.

NATS has fully cooperated with the independent panel review and engineering protocols were followed on the day, with staff situated on site and on call, a spokesman said in a statement.

Air-traffic controllers were forced to input flight plans manually, severely limiting the number of aircraft that could fly through the UK’s airspace.

“It is highly unlikely that the same unique set of circumstances will converge at a future point to cause a major outage,” the report said. “However, it is possible that a different set of factors could create a similar scenario and without improvements to resilience planning the impact could also be significant.”

The report said airlines incurred “considerable costs” to cover consumers following the glitch, with British Airways spending £18.3 million ($23.4 million) and Ryanair Holdings Plc paying out £15 million. Chief Executive Officer Michael O’Leary has since repeatedly called for NATS’ chief executive Martin Rolfe to resign.

“The fact that key NATS engineers were sitting at home during one of the peak travel weekends, combined with findings that NATS has a fundamental lack of pre-planning, documentation, and coordination, clearly demands senior management changes,” O’Leary said in an emailed statement on Thursday.

A final report on the incident is expected to be published later this year.

(Updates with statements from NATS and Ryanair.)

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