The leader of the influential nonprofit that raises the bulk of Central Park’s budget told supporters Thursday he had “significant concerns” over Mayor Bill de Blasio’s plan to build stables for horse-drawn carriages within the green space.

“We have not been engaged in the development or evaluation of the plan,” Central Park Conservancy president Doug Blonsky wrote in an email to the organization’s backers. “As the leading advocate for Central Park, we want you to know that we have significant concerns with the proposal.”

De Blasio countered that the park “belongs to the people,” and he and the City Council are the ultimate decision-makers.

The mayor, Council Speaker Melissa Mark-Viverito and the union representing the carriage drivers have reached a tentative compromise that would reduce the number of working horses from 180 to 75, with the animals living in a park stable to be built by the city by 2018.

De Blasio has said his team is looking at refitting a city building near the 85th Street Transverse at an estimated cost of $25 million.

The council is expected to vote on the bill Feb. 5.

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At a hearing last week, council members grilled de Blasio aides who couldn’t provide logistics of the proposal.

“It was clear that there are still many unresolved issues,” Blonsky wrote of the hearing, adding that he believes “the many joggers, walkers, and dog walkers currently finding peace” along the park’s bridle path would be disrupted by the carriages.

De Blasio said that conservancy officials have been consulted “throughout this process.”

“The conservancy provides a lot, but the park belongs to the people,” said the mayor, whose 2013 campaign was boosted by an animal welfare group backing a ban on the carriage industry.

The council is also set to decide Feb. 5 on pay raises for themselves and other elected officials, and a good-government group questioned the coupling of the carriage and salary hike votes.

“We hope that there has been no ‘horse-trading’ going on between these two bills, given that one is strongly sought by the mayor and the other one by the council,” Dick Dadey of Citizens Union said.