City Council Speaker Julie Menin is threatening to have the Big Apple join a lawsuit against a luxury events venue under the Ed Koch-Queensboro Bridge for allegedly hijacking a public plaza.

“We must ensure that this public space remains genuinely accessible to all New Yorkers and that the legal commitments governing this space are enforced,” Menin said in a Sept. 3 letter to Louis Rose, chairman of Rose Group Hospitality, which operates the Guastavino’s venue in the plaza under the iconic span.

A recent suit alleged that Guastavino’s has illegally claimed the public city plaza as its own for the past decade — repeatedly excluding citizens from the space — while charging venue customers up to $300,000 a pop to rent it for private events.

The city-owned plaza — part of a massive parcel first purchased in 1900 for the construction of the bridge — was meant to be “a sitting area accessible to the public,” the suit said.

But the venue’s bosses decided they’d rather act like they owned the place, according to court documents.

They have illegally blocked taxpayers from it using fake signs and security guards — and sometimes even local cops — while hawking the spot for posh weddings and galas, the filing said.

The lawsuit — brought by a Brooklyn mentalist, a Manhattan grad and a veterinary technician — demands a judge formally declare the plaza public land and force the venue to hand over its ill-gotten gains.

Menin, who represents the Manhattan neighborhood around the bridge, has demanded a meeting with the ownership of the Rose Group/Guastavino’s and threatened to have the council support the lawsuit with an amicus brief if the dispute isn’t resolved.

“Only after public outcry and an inquiry by my office to City agencies has Guastavino’s subsequently taken steps to clarify that the plaza is open to the public,” Menin said in the letter.

“Given the history of this site and the Council’s role in the City’s land use process, it is important that there be a clear and durable understanding regarding the plaza’s future operation, including public access, signage, and security practices.

“The City Council is considering filing an amicus curiae in the pending litigation, specifically with respect to the failure to maintain a public plaza associated with City-owned property and the integrity of commitments made through the City’s land use and disposition process,” the top councilwoman said.

She said she is requesting a meeting with Guastavino’s to see if the space dispute can be resolved before deciding whether to support the suit against the events planner.

Guastavino’s/The Rose Group did not comment to The Post.

But the venue recently told amNY.com that it “acknowledges that this is a public plaza’’ and that it is working with the city to “come up with a path forward.”