The Trump administration will pause the permitting process for a controversial gold and copper mine in Alaska on Monday on the grounds that the firm must do more to address how it will harm the environment, according to three individuals briefed on the decision who spoke on the condition of anonymity to discuss internal deliberations.

The move represents a major blow to Pebble Mine, which was vetoed under Barack Obama but has been revived under the Trump administration. Several high-profile Republicans, including the president’s eldest son, Fox News host Tucker Carlson and Vice President Mike Pence’s former chief of staff Nick Ayers, have campaigned against the project on the grounds it could harm the world’s largest sockeye salmon fishery in Bristol Bay.

The U.S. Army Corps of Engineers will send a letter to the Pebble Limited Partnership on Monday saying because the plan will cause “significant degradation” the company will have to do more to address its environmental impact before it can receive a key federal permit.

The Corps concluded late last month that the operation would have “no measurable effect” on the area’s fish populations but also found it would permanently destroy 2,200 acres of wetlands and 105 miles of streams in the region.

Politico first reported the decision to delay the permit Saturday afternoon.

Asked about the matter, the White House referred any questions to the Corps. The agency did not respond to a request for comment.

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Pebble Limited Partnership CEO Tom Collier said in a phone interview Saturday that his company anticipated the Corps would demand more details on how it would address the operation’s affect on the surrounding area. The company will have to identify wetlands it can preserve within the specific watershed of where the mine will be built, he said.

“They’re going to expect a substantial amount of mitigation. But we’ve known that for a months … That’s something we expect can be dealt with in due course,” Collier said, adding he remains confident his company can address the agency’s outstanding concerns. “I continue to deny that there is any basis for a story that the White House has decided to kill the project.”

But some White House officials have expressed reservations about granting the mine, which would be the largest in North America, a “record of decision” for months. That federal permit marks a decisive step for the company, which then plans to apply for state permits to begin construction.

Donald Trump Jr., who has fished in Bristol Bay, urged his father to block the mine. So has environmental philanthropist Andrew Sabin, a Trump donor, as well as Carlson, who ran a segment on his show criticizing the project earlier this month. Ayers, Pence’s former chief of staff, also played an instrumental role lobbying senior administration officials to reject the proposal.

The Theodore Roosevelt Conservation Partnership has been running ads on Fox News and social media against the project in an effort to attract the attention of Trump and his supporters. Now the company plans to launch its own ad campaign Tuesday. Collier said he did not know exactly where the commercials would air, but they would “run on Fox News.”

Conservation groups, for their part, welcomed the news that the mine faces a steeper hurdle in winning the administration’s approval.

“Pebble Mine has always been the wrong mine in the wrong place,” said Collin O’Mara, president and CEO of the National Wildlife Federation, in an email.