A new ruling by the European Court of Justice could force airlines to pay countless sums in compensation for flights cancelled or subject to long delays caused by a strike.

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 not liable to the compensate for delays.

The court disagreed with TUIfly, saying similar circumstances were only applicable if they were not part of the normal activity of the airline and were beyond its control. The judges ruled that internal restructuring was part of a normal business practice that brought potential risks during the negotiating process and required enough due diligence by the company to take into considering the possibility of a strike.

“The current ‘wildcat strike’ case cannot be regarded as being beyond TUIfly’s actual control,” the Court of Justice said.

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

Airlines whose operations are disrupted by strikes are already required by Eurpean law to care for the passengers by paying for meals and accommodation. In years past, the general consensus was that forcing the airlines to compensate passengers would give trade unions an unreasonable amount of negotiating power.

The airlines were later given an exemption from paying compensation if they can prove that “extraordinary circumstances” played a part in affecting the schedule of regular flights.