The city’s powerful teachers union and some parents are concerned that proposals for charter schools to share space with city-run public schools could undermine a new state law limiting class size.

United Federation of Teachers President Michael Mulgrew said he believes recent proposed “co-locations” of charter schools with traditional public schools signal Mayor Eric Adams is taking a more accommodating approach to charters than his predecessor. Charters are publicly funded, privately run and not bound by the same rules as traditional public schools.

“We haven’t had this many [co-location] proposals done this quickly in quite some time,” said Mulgrew. “So clearly this administration has decided they want to work with some of the people in the charter school industry to get them the free space to the detriment of the children in the public schools.”

The prominent charter network Success Academy Charter Schools has three proposed co-locations up for review in the coming months, public records show.

“The surplus of unused public space — 140,000 unused seats in NYC — makes co-location fiscally responsible public policy and allows parent needs to be met more quickly, as opposed to private real estate, which is costly and can take years,” Success Academy spokesperson Ann Powell said.

Last Wednesday, the Panel for Education Policy granted Success Academy permission to share a building with the Waterside School for Leadership, a middle school in Rockaway, by the slim margin of 8-7. A majority of the panel is appointed by the mayor.

The panel's Vice President Tom Sheppard, who voted against the co-location, said he worried there is a rush to approve such moves before a new law requiring smaller class sizes in traditional public schools goes into effect. Under the law, the city must cap classes at 20 to 25 students, depending on the grade level, over a five-year period beginning next school year.

Sheppard and the teachers union argue that smaller classes mean schools will need more classrooms. They say allowing more charters into buildings with traditional public schools will limit the ability to expand classes to comply with the law.

“There’s a rush to make these proposals now before these changes in utilization happen,” Sheppard said in an interview. "I want this whole process to slow down.”

Michael Mulgrew, president of the teachers union, said the Adams administration had decided to give charter schools free space "to the detriment of the children in the public schools.”

Teachers in charter schools are generally not unionized, and unions typically oppose their growth, arguing they siphon off resources from the rest of the school system. Co-locations are particularly controversial because schools often see themselves as competing for resources and space.

Adams has long been supportive of charter schools. In April, he welcomed former Mayor Michael Bloomberg back to City Hall to announce a $50 million donation to fund a summer program for charter school students.

“Definitely [the administration] is set up to be charter-friendly,” said Sheppard.

Education department spokesperson Art Nevins countered that the agency was simply following state law requiring that new or growing charter schools receive space or help with rent from the city and said the number of co-locations up for consideration is in line with previous years.

But charter critics are bracing for the possibility of a renewed battle under Adams. The charter debate peaked under Bloomberg, who had a hostile relationship with the teachers union. It then died down under former Mayor Bill de Blasio, who courted labor unions as key political allies.

A hearing last week on the co-location of Success Academy with the Waterside School on Beach 110th Street evoked memories of Bloomberg-era conflicts. As part of the plan, Waterside’s elementary school will move into another building nearby.

Dozens of students and parents from Success Academy spoke in favor of the proposal, while students and parents from Waterside were opposed.

“Our school lives will be changed,” said Waterside seventh grader Sophia Wright, who worried about overcrowding.

Education department officials said the co-location plan will result in less overcrowding.

“As a taxpayer, I believe my kid deserves the same advantages in school space as any other,” Success Academy parent Claudette Hall said.

Mulgrew said co-locations breed resentment.

“Parents are adamant that they do not want to be in co-located schools,” he said. “They feel very strongly their children are being treated very differently when there’s charter schools co-located in their buildings.”