Gregory, in the Aug. 6 decision, said construction of the project cannot proceed without a valid and enforceable permit from FWS for the “incidental taking” of five threatened or endangered species in the pipeline's path.

Without valid permits from the two federal agencies, the ACP, “should it continue to proceed with construction,” would violate the terms of the project's approval from the Federal Energy Regulatory Commission, Gregory said.

Environmental organizations that brought the two cases covered by the opinion have called on the FERC to halt construction of the pipeline along its entire length through West Virginia, Virginia and North Carolina.

The panel's ruling on the “incidental taking” of threatened or endangered species expands on its decision in May to vacate the FWS permit for construction that would harm habitats for the clubshell mussel, the rusty-patched bumblebee, the Madison Cave isopod, the Indiana bat and the Northern long-eared bat.

Gregory said the NPS also failed to show that it had the authority to grant a right-of-way through the parkway area. The pipeline would require a 125-foot right-of-way, which opponents argue would spoil scenic views.

A separate lawsuit is challenging NPS approval of the project's plan to drill through the Blue Ridge Mountains beneath the Appalachian Trail without first obtaining approval from Congress.

“What is clear from today, there is a 600-mile pipeline with a hole in the middle,” said DJ Gerken, an attorney for the Southern Environmental Law Center in Asheville, North Carolina, who argued the two cases for Defenders of Wildlife, the Sierra Club and the Virginia Wilderness Committee.

“What they should be doing right now is rethinking the whole project,” Gerken said of the ACP, led by Dominion Energy. “They should at least be rethinking a new route.”

Dominion took a completely different view of the panel's decision and expressed confidence that the two agencies can quickly reinstate the permits to satisfy the court's concerns in both cases.

“We believe the court's concerns can be promptly addressed through additional review by the agencies without causing unnecessary delay to the project,” spokesman Aaron Ruby said in a statement on Monday. “In the meantime, we will continue making progress with construction in West Virginia and North Carolina.”