Conservationists are outraged by Interior Department Secretary Ryan Zinke’s proposal—as outlined in a memo to President Donald Trump that was leaked by media outlets late Sunday—to revoke or alter federal protections for at least 10 national monuments and expose protected lands and waters to commercial activity.

“If President Trump accepts Zinke’s advice, and moves to eviscerate monument protections, he’d be ignoring the law—and the will of the American people,” said Natural Resources Defense Council (NRDC) president Rhea Suh, who also vowed to fight the changes in court, should the president act on the secretary’s suggestions. Suh was not alone in her charge that it would be illegal for the president to make the proposed changes.

“Acting on these recommendations would represent an unprecedented assault on our parks and public lands, and undermine bipartisan progress to protect our lands and waters that dates to Theodore Roosevelt,” said Jamie Williams, president of the Wilderness Society. “We believe the Trump administration has no legal authority to alter or erase protections for national treasures.” Williams also promised to battle “these illegal and dangerous recommendations” in court, if Trump acts on them.

In April, the president ordered Zinke to review 27 monuments—all larger than 100,000 acres—that have been granted federal protections under the Antiquities Act since 1996, claiming that his predecessors went too far when designating certain U.S. lands and waters as protected from commercial activity. Zinke’s leaked 19-page memo (pdf) follows a two-page summary (pdf) the department released in late August, which also garnered intense criticism from conservationists.

“It appears that certain monuments were designated to prevent economic activity such as grazing, mining, and timber production rather than to protect specific objects,” the memo reads. Although it notes that “comments received were overwhelmingly in favor of maintaining existing monuments,” the vocal resistance to rolling back protections for national monuments seems to have fallen on deaf ears.