BRUSSELS, Feb 12 (Reuters) - Ryanair warned on Thursday that Dublin Airport may be forced to cut capacity by over 15% and Aer Lingus flights to New York could be blocked after an adviser to Europe's top court backed the basis of a passenger cap at Ireland's main airport.

The imposition of the 32 million passenger-per-year cap has been suspended pending a ruling by the Court of Justice of the European Union, but on Thursday the court's Advocate General Campos Sanchez-Bordona indicated there was no reason to strike down the cap. 

The court, which follows such non-binding opinions in four out of five cases, will rule in the coming months.

"Today's Advocate General Ruling, which is likely to be followed by the ECJ in June or July, means that traffic at Dublin Airport, which will be 38 million in 2026, must now be cut back to 32 million, if the Irish government doesn't take urgent action," Ryanair, one of the airlines that brought the case, said in a statement.

The Irish cabinet gave formal backing this week to a law to scrap the passenger cap. Infrastructure Minister Jack Chambers said on Thursday that the legislation would likely be enacted in the early summer.

"Notwithstanding the Advocate General's opinion today, the Irish government is determined to get on with it and lift the cap," he told RTE radio.

The cap was imposed by local planners in 2008, but only became an issue in 2024 when passenger numbers neared the limit. Dublin Airport overshot by over 4 million passengers last year.

In the challenge, Ryanair and other airlines argued that the elimination or reduction of slots was unjustified and compromised their freedom to conduct business, but Thursday's opinion rejected both arguments.

U.S. industry trade group Airlines for America filed a complaint with the U.S. Department of Transportation last month asking it to curtail Irish carriers' access to the U.S. if the cap at Dublin Airport is not swiftly scrapped.

That could significantly hurt Aer Lingus, the former flag carrier that is part of the IAG group of airlines and the only Irish airline with significant U.S. operations, something O'Leary described as a "real risk".

(Reporting by Foo Yun Chee, Sudip Kar-Gupta, Conor Humphries and Padraic Halpin; editing by Inti Landauro, Sharon Singleton, William Maclean)