BRUSSELS — Fallout from a cyberattack that disrupted check-in systems at several European airports extended into a second full day on Sunday, as passengers faced dozens of canceled and delayed flights — and the impact was poised to worsen for at least one major airport.

Brussels Airport, seemingly the hardest hit, said it asked airlines to cancel nearly 140 departing flights scheduled for today because a U.S.-based software system provider “is not yet able to deliver a new secure version of the check-in system.” The airport said 25 outbound flights were canceled Saturday and 50 on Sunday.

Starting late Friday, airports in Berlin, Brussels and London were hit by disruptions to electronic systems that snarled up check-in and sent airline staffers trying options like handwriting boarding pa

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