Europe’s airspace coordinator said a trade union representing workers at its operations center had warned of possible strikes over a period of six months, potentially creating more disruption as the busy summer 2023 travel season reaches its peak.

Eurocontrol described the protests as a “pre-warning” in a statement and said it was engaging in dialogue with the Union Syndicale Bruxelles to end the possible industrial action.

A strike could delay or cancel as many as 12,600 flights this summer across Europe each day, The Times reported earlier July 7, citing an industry source. That would amount to about a third of all flights as Eurocontrol predicts about 33,000 flights each day between July and August 2023.

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Eurocontrol didn’t provide a time frame for when possible strikes may occur and said it was too early to speculate on any impact that strikes may have.

Eurocontrol’s network management operations center receives 96,000 messages a day and oversees more than 10 million flights per year. It works with airports, users of civil and military airspace and other aviation groups in Europe to reduce air traffic disruptions and ensure smooth operations.

Air traffic controller strikes not only affects flights to and from the country where they take place but also lead to turmoil elsewhere in Europe due to delays for planes that have to fly over that country to reach other destinations.

The airline industry is already grappling with strikes by air traffic controllers in France, which have led to thousands of flight cancellations since the start of 2023, further denting the image of a country that’s been wracked by protests and riots. Ryanair Holdings Plc said it was forced to cancel 900 flights in June 2023 because of these strikes alone.

A single walkout among Geneva airport personnel, including air traffic controllers, that lasted a few hours in June grounded flights affecting about 8,000 passengers.

Read more: JetBlue Joins United in Shifting Blame to FAA for Delays

In Germany, Deutsche Lufthansa AG is also seeking to avert disruptions caused by a possible pilot strike as talks over work conditions continue. Persistent staffing shortages have also hit the sector as demands for flying boom post-pandemic.

Eurocontrol warned earlier in July that passengers flying to vacation hot spots like Marseille or Athens face disruptions because, while travel has returned to pre-pandemic levels, it’s putting additional strain on the system as airlines and ground services still grapple with staffing shortages.

Although airports are predicting smoother operations, staffing issues could put pressure on hubs, especially if there are network disruptions, Eurocontrol said at the time.