Three environmental groups have appealed a judge’s decision to allow construction on a 145-mile transmission line through western Maine, but the project will not immediately be delayed by that legal action.

Sierra Club Maine, the Natural Resources Council of Maine, and the Appalachian Mountain Club sued the U.S. Army Corps of Engineers this fall in hopes of blocking the corridor. They alleged the Corps conducted a flawed and inadequate environmental review for the New England Clean Energy Connect project, and they asked a federal judge to issue a preliminary injunction that would have prevented construction while they fought in court.

But U.S. District Judge Lance Walker denied that request earlier this month, which means Central Maine Power could begin construction on the $1 billion project as soon as next month. So the plaintiffs have asked the First Circuit Court of Appeals in Boston to reconsider that decision. They also asked Walker to consider granting an injunction while the higher court reviews their appeal, and he again denied that motion. So work on the project could begin while the legal challenges are still pending.

The plaintiffs filed an emergency motion in their appeal Wednesday evening. They argued in part that Walker took too narrow an approach in his ruling and did not consider all the potential harms from the project. They also asked the court to grant an injunction by Jan. 15 in light of the possible impending start to construction.

“These harms are not imagined, speculative, or remote; there is no dispute that the cleared corridor and related infrastructure are permanent or at least of long duration,” their appeal said.

The defendants will have the chance to respond to that motion. CMP is an intervenor in the case and could have the chance to file its own argument in the appeal. The utility revealed in previous court filings that a six-month delay in bringing the New England Clean Energy Connect online would cost an additional $31 million, while a one-year delay would raise that figure to $37 million.

A spokesman provided a written statement on behalf of Thorn Dickinson, CEO and president of project developer NECEC LLC.

“The court conducted a thorough analysis regarding the request for an injunction of the permit issued by the Army Corps of Engineers and it was denied,” that statement said. “This is just another desperate effort led by fossil fuel companies to delay the project. The New England Clean Energy Connect continues to meet every permitting milestone throughout the regulatory process. We need to allow time for some planning matters such as the transfer of the project from CMP to NECEC but look forward to breaking ground in the coming weeks.”

A spokesman for the NRCM said the legal effort is led by the three environmental groups, not fossil fuel companies.

To cross the Canadian border, the project still needs to obtain a presidential permit from the federal Department of Energy. There is still no specific start date for construction, but developers have said it could begin in January at the earliest.

The NECEC project would transmit 1,200 megawatts of hydroelectricity from Canadian utility Hydro-Quebec to the New England grid, and would be funded by Massachusetts ratepayers. The 145-mile-long transmission corridor would cut through about 53 miles of mostly-commercial forest owned or controlled by CMP in western Maine before following existing transmission corridors. The environmental groups asked the court for an injunction only related to that segment to prevent the clearing of trees there.

The Sierra Club, Appalachian Mountain Club and National Resource Council of Maine have said that the 53 miles of new power line would “forever fragment the largest contiguous temperate forest in North America and perhaps the world. The destruction would clear trees and plants through wetlands and across streams, impacting important deer wintering habitat and other areas where public agencies and private citizens have spent millions of dollars to protect habitat for Maine’s brook trout.”