U.S. energy company Equitrans Midstream Corp's long-delayed $6.6 billion Mountain Valley natural gas pipeline from West Virginia to Virginia received another negative court ruling on Friday.

The U.S. Court of Appeals for the District of Columbia Circuit ruled the U.S. Federal Energy Regulatory Commission (FERC) "inadequately explained its decision not to prepare a supplemental environmental impact statement."

The statement would address "unexpectedly severe erosion and sedimentation along the pipeline’s right-of-way."

The Court remanded the case back to FERC to either "prepare a supplemental environmental impact statement or to better explain why one is unnecessary."

The case was brought by the Sierra Club and other environmental groups opposed to the project.

"We see this ruling as creating an additional hurdle to restarting construction when (Mountain Valley) reacquires its outstanding permits this year, as it could force a longer review ... and could push the project’s completion into 2024," said analyst at ClearView Energy Partners, a research firm.

Equitrans said it was reviewing the decision, without providing any additional details. Its stock rose about 4% to a 12-week high of $6.14 per share in late Friday trading.

In early May, it said it could finish the pipe by the end of 2023, but noted "there remains significant risk and uncertainty, including regarding current and likely litigation."

The U.S. Fourth Circuit Court of Appeals, which has vacated several of the project's federal and state permits - some more than once - is still hearing lawsuits, including a challenge against authorizations from the U.S. Fish and Wildlife Service.

Equitrans is waiting for new authorizations from the West Virginia Department of Environmental Protection, the U.S. Army Corps of Engineers and FERC.

In addition to pursuing permitting the regular way, Equitrans said it "continues to support the potential enactment of federal energy infrastructure permitting reform legislation that specifically requires completion of the (Mountain Valley) project."

When Mountain Valley construction started in February 2018, Equitrans estimated the 303-mile (488-km), 2-billion-cubic-feet-per-day (bcfd) project would cost about $3.5 billion and enter service by late 2018.

Mountain Valley is owned by units of Equitrans, NextEra Energy Inc, Consolidated Edison Inc, AltaGas Ltd and RGC Resources Inc.

(Reporting by Scott DiSavino; Editing by Marguerita Choy)