Work on the Atlantic Coast Pipeline (ACP) has again been delayed after the U.S. Court of Appeals for the Fourth Circuit on Monday stayed federal authorizations that could continue to slow a project that would move 1.5 Bcf/d of natural gas from Appalachian shale fields.

The court’s decision prevents work that was poised to restart along 20 miles of the 600-mile route.

It’s the third time in four months that the Fourth Circuit has vacated or stayed federal authorizations for the project. This time, the appeals court stayed decisions by the U.S. Forest Service (USFS) allowing tree clearing, blasting and trenching in the George Washington National Forest and the Monongahela National Forest in Virginia and West Virginia. The court stayed the authorizations to review a challenge filed in February by the Southern Environmental Law Center (SELC) and the Sierra Club on behalf of several regional environmental organizations.

The stay was granted on Monday, a week after FERC lifted a stop worker order for the entire project that had been in effect for more than a month. The Federal Energy Regulatory Commission stopped all work in August after the Fourth Circuit vacated key permits issued by the U.S. Fish and Wildlife Service and the National Park Service. The Commission lifted the order once those agencies issued revised permits.

Given the Fourth Circuit’s latest decision, ACP could face a similar situation affecting the entire project.

The SELC filed at FERC on Tuesday asking the Commission to again suspend work along the entire route, noting that the project is no longer in compliance with its certificate because it does not have all authorizations required under federal law. Reaching for a possible precedent set by the first stop work order, the center pointed out that the court found that the environmental groups are likely to succeed on the merits of their challenge.

SELC’s filing evoked FERC’s language, included in the August stop work order justifying the decision, to say that if its challenge were to be successful, the project’s backers may need “to revise portions of the ACP route after additional review by the Forest Service.” As FERC did last month, the SELC suggested that continued construction would unnecessarily expend resources for facilities that might ultimately be relocated.

“FERC should stop construction elsewhere until these issues are resolved, to avoid wasting ratepayer dollars building a route that may not be viable,” said SELC senior attorney DJ Gerken in a statement.

According to the SELC, tree clearing had been scheduled to restart in the forests on Monday. ACP said in response it wasn’t backing down, sounding a resilient tone by saying that it stands behind the USFS review and is prepared to “vigorously defend” it during oral arguments that are scheduled for Friday.

“Make no mistake, we will complete this project for the good of our economy and the environment,” said spokesman Aaron Ruby. He said public utilities are relying on the pipeline to meet growing demand in the Southeast. “Opponents’ delay tactics will not stop this project…They will only drive up consumer energy costs, delay the transition to cleaner energy and make it harder for public utilities to reliably serve their customers.”

ACP applied for a special use permit from the USFS in June 2016. SELC argues that construction would cause such significant impacts that the project can’t comply with standards designed to protect the national forests. The USFS waived compliance with those standards in a Decision of Record and issued a special use permit for the project in November 2017. The decision and permit are being challenged by the environmental groups.

While the stay applies to a small stretch of the pipeline, the environmental groups said in court documents the work would impact more than 400 acres of the national forests.

Work continues on the remainder of the pipeline, Ruby said. The related Supply Header Project (SHP), which recently received more notices to proceed with construction from FERC, is also not affected. SHP would include roughly 37.5 miles of pipeline looping and modifications to existing compressor stations in West Virginia and Pennsylvania.

ACP would originate in West Virginia, pass through Virginia and into North Carolina to move more Appalachian gas to the Southeast. The project, backed by Dominion Energy Transmission Inc., Duke Energy Corp., Piedmont Natural Gas and Southern Company Gas, is still targeting a 2019 in-service date.