Airlines for America has submitted a formal complaint to the US Department of Transportation, arguing that Ireland is in breach of its legal obligations because of what it describes as Dublin Airport’s illegal passenger traffic cap.

In its filing, the airline trade body warned that the cap could force some US carriers to lose their existing slot rights at Dublin Airport, while also blocking other US airlines from expanding services and passenger numbers on routes to Ireland’s main aviation hub.

The complaint highlights that the restrictions remain in place despite Dublin Airport having opened a second runway and possessing capacity to handle up to 60 million passengers per year. Airlines for America said the cap is therefore unjustified and harmful to transatlantic connectivity and competition.

The move follows a similar legal challenge last year by several airlines, including Ryanair and Aer Lingus. That case resulted in a stay on the implementation of the cap and its referral to the European Court of Justice.

Ryanair chief executive Michael O’Leary said the ongoing failure of the Irish government to resolve the issue has now escalated beyond Europe. He argued that the situation has become “an international embarrassment for Ireland” not only before the European courts, but also in the eyes of the US administration.

O’Leary also criticised Irish Prime Minister Micheál Martin, saying the continued delays amount to a failure to deliver on government promises to address capacity constraints at Dublin Airport.

The complaint raises the prospect of increased political and legal pressure on Ireland as the dispute over Dublin Airport’s traffic limits increasingly draws in US regulators and airlines, alongside ongoing scrutiny from European institutions.