Several cases were brought to the EU's Court of Justice after German airline TUIfly said a 2016 wildcat strike - one not formally initiated by a trade union - was an extraordinary event, and so it was not liable to compensate for delays.

The strike followed the company's announcement of a restructuring plan.

EU regulation gives passengers on a delayed flight the right to compensation of up to 600 euros ($745), depending on the distance of the flight.

Airlines are exempt from paying this if they can show there were "extraordinary circumstances" which they could not have reasonably avoided.

The court disagreed with TUI, saying such circumstances were only applicable if they were not part of the normal activity of the airline and were beyond its control.

Judges said restructuring was part of normal business, with the foreseeable risk of disagreements and conflict with staff.

"The 'wildcat strike' at issue in the present case cannot be regarded as beyond TUIfly's actual control," the Court of Justice said.

The dispute, which saw staff placing themselves on sick leave, was brought to an end by an agreement in October 2016.

The court added that making a distinction between wildcat strikes and those organized by a trade union would make passenger compensation dependent on the rules in different EU countries, which could undermine air passenger rights.

(Reporting by Robert-Jan Bartunek; Editing by Philip Blenkinsop and Mark Potter)