(Adds Justice Department stance, stock closing price)
July 11 (Reuters) - U.S. energy company Equitrans Midstream said on Tuesday it is evaluating all legal options, including an appeal to the U.S. Supreme Court, after an appellate court halted construction of the West Virginia-Virginia Mountain Valley natural gas pipeline in the Jefferson National Forest.
Equitrans shares closed down about 4.8% at $8.75 - near a six-week low - a day after the Richmond, Virginia-based 4th U.S. Court of Appeals agreed with arguments made by groups opposed to the project that construction should stop as the court reviews federal approvals that allowed work to begin.
Approval for the 303-mile (488-km) project was placed into the text of a federal law called the Fiscal Responsibility Act, which ended the debt ceiling crisis.
Equitrans said in a statement the 4th Circuit's decision "defies the will and clear intent of a bipartisan Congress" and President Joe Biden's administration. The pipeline, which had received authorization from the U.S. energy regulator last month to restart construction, is considered key to unlocking more gas supplies from Appalachia, the nation's biggest shale gas-producing basin.
"We are evaluating all legal options, which include filing an emergency appeal to the U.S. Supreme Court. Unless this decision is promptly reversed, it would jeopardize Mountain Valley's ability to complete construction by year-end 2023," the company said.
Mountain Valley is owned by units of Equitrans, the lead partner building the pipe, NextEra Energy, Consolidated Edison, AltaGas and RGC Resources.
Analysts at ClearView Energy Partners, a research firm, said Mountain Valley had planned to resume construction on June 12 if the project had not been held up by court orders.
The U.S. Justice Department had filed briefs urging the 4th Circuit to dismiss the appeals challenging Mountain Valley.
ClearView said that "the Biden administration did not equivocate in its view that the cases must end," and added: "This strengthens our view that the cases will be dismissed, if not by the 4th Circuit then by the Supreme Court."
Mountain Valley is one of several U.S. projects that have been delayed by regulatory and legal fights with environmental and local groups in recent years.
Even though the 4th Circuit decision is "only applicable to the three miles (4.8 km) of the Jefferson Forest," analysts at Height Capital Markets, an investment banking and research firm, said the decision could jeopardize a 2023 in-service date.
When Mountain Valley started construction in February 2018, Equitrans estimated the 2.0-billion cubic feet per day project would cost about $3.5 billion and enter service by late 2018. (Reporting by Sourasis Bose in Bengaluru and Scott DiSavino and Clark Mindock in New York; Editing by Wilt Dunham, Nivedita Bhattacharjee and Chizu Nomiyama)