Travelers wait in Terminal 4 at Heathrow Airport on Saturday.JUSTIN TALLIS / AFP - Getty Images

Some of Europe's biggest airports, including the region's busiest, London's Heathrow, raced to restore normal operations on Sunday after hackers disrupted automatic check-in systems.

Hackers on Saturday targeted check-in and boarding systems provider Collins Aerospace, owned by RTX, disrupting operations at Heathrow, Berlin Airport and in Brussels, where passengers faced long queues, cancellations and delays.

Disruption had eased significantly by early Sunday despite some continuing delays, according to airport officials and data, while regulators said they were investigating the source of the cyberattack.

Brussels Airport said that 45 out of 257 scheduled departing flights on Sunday were cancelled but operational adjustments had kept waiting times down. "There are delays between 30 and 90 minutes," a spokesperson for the airport told Reuters, adding it was not clear yet how much longer disruptions would last.

A passenger flying from Brussels said disruption had been minimal.

"For me, it was business as usual. For those poor souls who didn't do online check-in or have bags to check, they may be waiting a bit," he said.

The disruption is the latest in a string of hacks to hit sectors from healthcare to autos. A breach at carmaker Jaguar Land Rover halted production, while another caused Marks & Spencer losses in the hundreds of millions of pounds.

RTX called the incident a "cyber-related disruption" and said it had impacted its MUSE software, which is used by several airlines. RTX was not immediately available for comment on Sunday.

Berlin Brandenburg Airport said on Sunday problems persisted but that it was working with the company to resolve the issue. A manual workaround was in place and there were currently no major delays or cancellations, it added.

Heathrow said early on Sunday work was continuing to recover from the check-in system outage. It added that "the vast majority of flights have continued to operate".

An analysis by aviation data provider Cirium said delays at Heathrow were "low," Berlin had "moderate" delays, while Brussels had "significant" delays, but were decreasing.

Airports race to fix check-in glitch after cyberattack hits Europe