Ontario First Nation chiefs are demanding Doug Ford's government return land to the protected Greenbelt that the province removed for development.
The Chiefs of Ontario have voted unanimously in an emergency meeting to oppose the land swap.
They say the deal violates the Williams Treaties that were settled in 2018.
The chiefs say they were not consulted on the land swap that involves land on their territory.
Earlier this month, the auditor general found the Ford government gave preferential treatment to certain land developers when it decided to remove 7,400 acres in 15 different areas of the Greenbelt, while adding 9,400 acres elsewhere, in order to build homes.
Ford has said he has done nothing wrong in the fiasco that has embroiled his government since last year.