The developer of a $1 billion power transmission line through western Maine said Friday that it will temporarily suspend construction on the project while a legal challenge plays out in court.

The announcement came on the same day that Gov. Janet Mills wrote to the company to ask it to stop development in deference to state voters who roundly rejected the project on the November ballot. Maine voters approved Question 1, which was designed to block the corridor, by a roughly 60 percent to 40 percent margin.

Mills supports the project, known as New England Clean Energy Connect, but its future remains uncertain. Central Maine Power Co. parent company Avangrid and its subsidiary NECEC Transmission LLC have sued the state over the referendum. The Maine Department of Environmental Protection is also considering whether to suspend the project’s permit.

Thorn Dickinson, CEO and president of NECEC Transmission, said Friday that the ballot initiative “is and remains unconstitutional.” But he said the company decided to pause until the court acts upon its motion for a preliminary injunction, which seeks to prevent the new law from taking effect.

“This was not an easy decision,” Dickinson said in a statement. “Suspending construction will require the layoff of more than 400 Mainers just as the holiday season begins. It will also require the suspension of millions of dollars in future benefit payments being made to customers, businesses and host communities.”

Mills wrote to Dickinson on Friday with her request. She also certified the election’s results for the referendum, so Question 1 will become law in 30 days unless the legal challenge mounted by Avangrid is successful.

“While these matters are being considered by the DEP and the court, I believe CMP should give deference to the will of the voters,” Mills wrote. “On behalf of Maine people, I am asking you to honor their will by immediately halting any further construction on NECEC until the DEP and the court reach their independent conclusions. While you are not legally obligated to do so at this point, immediately halting construction in a voluntary manner will send a clear message to the people of Maine that you respect their will. I strongly urge you to do so.”

In his statement, Dickinson noted that electricity supply rates will go up more than 80 percent next year for most Maine customers. That increase is the result of climbing prices for natural gas, which is used to generate half of New England’s power.

“It is ironic that the day after our fossil fuel opponents were awarded a more than 80 percent raise, paid for by hard-working Mainers, we are now being forced to lay off workers and suspend benefit payments to the state,” he said. “It’s a valuable return on their more than $25 million political investment, while Mainers bear the environmental and economic cost.”

A spokeswoman for the governor’s office did not respond to an email Friday afternoon after the announcement from NECEC.

NECEC would bring power from Canadian hydroelectric producer Hydro-Quebec to the New England electric grid. The controversial project would connect to existing transmission lines in western Maine through a 53-mile stretch of working forest. It would mostly benefit Massachusetts customers, who are covering its cost.

Earlier this month, voters approved the referendum that bans construction of high-voltage power lines in the Upper Kennebec region and requires the Maine Legislature to approve any similar projects statewide retroactively to 2020. It also requires the Legislature, retroactive to 2014, to approve any such projects that use public land by a two-thirds majority.

The 145-mile NECEC route is on land owned or controlled by CMP, except for a one-mile patch through Maine public lands near The Forks. Two-thirds of the route follows existing CMP power line corridors, some of which are being widened up to 75 feet to accommodate another set of poles.

The day after the election, Avangrid filed its lawsuit to challenge the constitutionality of Question 1. The company also vowed to keep building the power line. In its complaint, Avangrid said almost $450 million already had been spent on the corridor, more than 80 percent of its right-of-way had been cleared and more than 120 structures had been erected. Avangrid, CMP and NECEC Transmission are all U.S. subsidiaries of Spanish energy company Iberdrola.

At the same time, opponents have been calling for a stop to construction and the Natural Resources Council of Maine asked the state to suspend NECEC’s construction permit in light of the vote. DEP Commissioner Melanie Loyzim has scheduled a hearing on that question for Monday.

“(Maine law states) the commissioner may revoke or suspend a license upon making certain findings, including a finding that: ‘There has been a change in condition or circumstance that requires revocation or suspension of a license,’ ” Loyzim wrote in her letter to the developer this month. “I have determined that the referendum result, if certified such that it will become law, represents an additional change in circumstance that may require suspension of the NECEC order.”

The commissioner is also considering whether to suspend the project permit based on an improper lease issued to cross one mile of public lands.

Pete Didisheim, advocacy director at the Natural Resources Council, said on Friday before NECEC agreed to halt construction that the environmental group appreciated the governor’s message. The project, he said, “is in serious jeopardy.”

“It’s time for CMP to respect the will of Maine people by abandoning this controversial project and restoring the portions of western Maine it has damaged,” Didisheim said in a statement.