(CNN) — The Biden administration has approved the massive Willow oil drilling project in Alaska, angering climate advocates and setting the stage for a court challenge.

The Willow Project is a decades-long oil drilling venture in the National Petroleum Reserve, which is owned by the federal government. The area where the project is planned holds up to 600 million barrels of oil, though that oil would take years to reach the market since the project has yet to be constructed.

By the administration's own estimates, the project would generate enough oil to release 9.2 million metric tons of planet-warming carbon pollution a year — equivalent to adding 2 million gas-powered cars to the roads.

The approval is a victory for Alaska's bipartisan congressional delegation and a coalition of Alaska Native tribes and groups who hailed the drilling venture as a much-needed new source of revenue and jobs for the remote region.

But it is a major blow to climate groups and Alaska Natives who opposed Willow and argued the project will hurt the president's ambitious climate goals and pose health and environmental risks.

The project has galvanized an uprising of online activism against it, including more than one million letters written to the White House in protest of the project, and a Change.org petition with millions of signatures.

Environmental advocates are expected to challenge the project in court. Earthjustice, an environmental law group, has been preparing a case against the project and intends to argue the Biden administration's authority to protect resources on Alaska's public lands includes taking steps to reduce planet-warming carbon pollution, which the Willow Project would ultimately add to.

Earthjustice President Abigail Dillen blasted the administration's decision on Monday.

"We are too late in the climate crisis to approve massive oil and gas projects that directly undermine the new clean economy that the Biden Administration committed to advancing," Dillen said. "We know President Biden understands the existential threat of climate, but he is approving a project that derails his own climate goals."

The venture was approved with three drilling pads instead of two. In recent weeks, the Biden administration had looked at reducing the number of approved drilling pads down to two and boosting nature conservation measures to try to assuage concerns climate and environmental groups had about the project.

But ConocoPhillips and Alaska's bipartisan congressional delegation aggressively lobbied the Biden White House and Interior Department for months to approve three drilling pads, saying the project would not be economically viable with two.

Ultimately, the administration felt they were constrained legally and had few options to cancel or significantly curtail the project — which was initially approved by the Trump administration. The administration determined that legally, courts wouldn't have allowed them to fully reject the project, two government sources familiar with the approval told CNN.

Many oil drilling leases on the site were decades-oil, which the administration felt gave ConocoPhillips certain existing legal rights. Reducing the drill-pads to two would have allowed the company to drill about 70 percent of the oil they were initially seeking.

Still, the final scope of the project will cover 68,000 fewer acres than what ConocoPhillips was initially seeking, the sources said.

CNN has reached out to ConocoPhillips for comment.

Biden on Monday is also expected to announce sweeping new protections for federal lands and waters in Alaska in tandem with Willow approval.

Biden will declare the entire US Arctic Ocean off limits to future oil and gas leasing and will announce new rules to protect over 13 million acres in the federal National Petroleum Reserve in Alaska from drilling. In all, the administration will move to protect up to 16 million acres from future fossil fuel leasing.

The protections will extend to the Teshekpuk Lake, Utukok Uplands, Colville River, Kasegaluk Lagoon and Peard Bay special areas — places that are important habitat to grizzly bears, polar bears, caribou and migratory birds. On Sunday, an administration official said the administration views the new actions as a "firewall" against both future fossil fuel leasing and expansion of existing projects on the North Slope.

Environmental groups lashed out at the Biden administration for its approval of Willow and said the increased protections for other Arctic regions will not undo the harm the project will cause.

Tiernan Sittenfeld, the senior vice president of government affairs for the League of Conservation Voters, said the league is "extremely disappointed" in the decision, calling the project "dangerous" and "dirty."

"This is in direct conflict with the Biden-Harris administration's goals of cutting climate pollution in half by 2030, and it's now all the more important they double down on executive action that maximizes climate and conservation progress," Sittenfeld said in a statement. "The new protections announced for the threatened Arctic are important, but they do not make up for Willow's approval."

Lena Moffitt, the executive director of Evergreen Action, which advocates for strong climate change policies, called the approval "an unacceptable departure from President Biden's promises to the American people on climate and environmental justice."

The Alaska Wilderness League, which works to protect Alaska's natural areas from industry and fossil fuel drilling, said it was "deeply disappointed" in the approval.

"This is the wrong decision for our climate future, for protecting biodiversity, and for honoring the frontline communities who have raised their voices against this project," said Kristen Miller, the executive director of the Alaska Wilderness League.

Sen. Martin Heinrich, a New Mexico Democrat, called the approval "disappointing."

"The western Arctic is one of the last great wild landscapes on the planet," Heinrich said in a statement. "Industrial development in this unspoiled landscape will not age well."

This story has been updated with more information.

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