President Donald Trump plans to shrink Bears Ears National Monument by 85 percent and reduce Grand Staircase-Escalante National Monument by half, according to documents obtained by The Washington Post that show the Utah sites would be cut even more than administration officials previously signaled.

Individuals briefed on the matter, who spoke on the condition of anonymity ahead of a formal announcement, cautioned that some changes still could be made before Trump makes his final decision public on Monday in Salt Lake City.

Trump will announce the changes to monuments established by former presidents Barack Obama and Bill Clinton, respectively, at the Utah Capitol before a crowd of supporters. The move will represent the most significant reductions by any president to designations made under the 1906 Antiquities Act, which gives the president unilateral authority to protect imperiled sites on federal lands and in federal waters.

The new proclamations, which also will split up both monuments into several smaller ones, would cut the overall size of Bears Ears from 1.35 million acres to 201,397 acres and Grand Staircase-Escalante from nearly 1.9 million acres to 997,490 acres.

Many Republicans, including Trump and state and local officials in Utah, have argued that previous presidents have abused their authority under the Antiquities Act by placing large areas off limits to industrial development, motorized vehicle use and other activities. Trump signed an executive order in April instructing Interior Secretary Ryan Zinke to scrutinize any national monument larger than 100,000 acres that was established in the last 21 years. His administration, Trump said, would "end these abuses and return control to the people, the people of all of the states, the people of the United States."

Zinke submitted a report to the White House in late August that proposed decreasing the size of at least four existing national monuments, plus changing the way a half-dozen more are managed, to allow for activities such as logging in forests where that is now off limits and commercial fishing in marine protected areas.

The Interior Department did not immediately comment on Thursday.

A coalition of conservation groups and tribes, who view Bears Ears as an important ancestral Pueblo site, are prepared to fight the changes in court. While Congress can alter national monuments easily through legislation, presidents have reduced their boundaries only on rare occasions.

Woodrow Wilson nearly halved the acreage of Mount Olympus National Monument, which Theodore Roosevelt had established six years earlier. In 1938, the U.S. attorney general wrote a formal opinion saying the Antiquities Act authorized presidents to establish a monument but did not grant them the right to abolish one. Several current legal scholars argue that Congress indicated in the Federal Land Policy and Management Act of 1976 that it reserved the right to alter any existing monument.

Kate Kelly, public lands director for the liberal think tank Center for American Progress, said in an email Thursday that the administration's plan would amount "to the largest elimination of protected areas in U.S. history" and would affect "an area more than six times the size of the Grand Tetons."

But many Republicans from western states welcome the idea of confining federal protections to a more limited area.

"The details of the president's announcement are his and his alone to make, but I appreciate his willingness to listen to my advice and, even more importantly, to give the people of Utah a voice in this process," Sen. Orrin Hatch, R-Utah, said in a statement. "I believe his proclamation, following Secretary Zinke's fair, thorough, and inclusive review, will represent a balanced solution and a win for everyone on all sides of this issue."

The draft documents do not include many specifics on how the revised monuments will be managed, but each emphasizes that the monument designations are "confined to the smallest area compatible with the proper care and management of the objects identified" by the president as worthy of protection.