The US government is set to allow the final phase of construction of the Dakota Access pipeline to begin as early as Wednesday, dealing a major blow to the Standing Rock Sioux tribe.

The decision sets the stage for a tense showdown at the site of the drilling, where indigenous and environmental activists have been camped for nearly a year.

The army corps of engineers provided notice of its intention to grant a permit for the oil pipeline to cross the Missouri river in North Dakota in a letter to congressman Raúl Grijalva, the ranking member on the House committee on natural resources. The decision follows Donald Trump’s executive order in his first week in office to expedite the project.

The letter, revealed in court filings, states that the easement will be issued “no earlier than 24 hours” after the delivery of the letter, which is dated 7 February. The letter also states that the army corps intends to waive the usual 14-day waiting period after congressional notification, meaning drilling could begin as early as Wednesday.

“The Obama administration correctly found that the Tribe’s treaty rights needed to be respected, and that the easement should not be granted without further review and consideration of alternative crossing locations,” said Jan Hasselman, an attorney for the Standing Rock Sioux tribe. “Trump’s reversal of that decision continues a historic pattern of broken promises to Indian Tribes and violation of Treaty rights. They will be held accountable in court.”

The decision marks a rapid reversal of the Obama administration’s decision to halt the $3.7bn pipeline, which the Standing Rock tribe says threatens its water supply and sacred indigenous sites. The Obama administration declined to issue the easement in December and initiated an environmental impact study, a process that could have delayed the project for years.

The Trump administration is also canceling the environmental impact study, according to the court filings.

“I have determined that there is no cause for completing any additional environmental analysis,” wrote Douglas W Lamon, the senior official performing the duties of assistant secretary of the army, wrote in a notice to the federal register.

The tribe has vowed to continue fighting the pipeline in court, but the approvals from the Trump administration, which has close ties to the pipeline corporation, could allow the project to soon be completed on the ground. The company has said in court that it will take about 60 days to finish.

The decision from Trump – who has invested in Energy Transfer Partners and accepted donations from the oil company’s CEO – comes nearly a year after Native American activists and environmental groups set up camps at Standing Rock to fight the project.

The 1,172-mile pipeline, which would transport crude oil from the Bakken oil field in North Dakota to a refinery near Chicago, inspired an unprecedented gathering of Native Americans and launched an international “water protector” movement.

Activists, known as ‘water protectors’, at the Standing Rock camp. The Trump administration has brought a sharp reversal of the Obama administrations’s decision to halt the pipeline. Photograph: Pacific Press/Rex/Shutterstock

In a statement, the Standing Rock Sioux tribe said it was “undaunted in its commitment to challenge an easement”, but Dave Archambault II, the tribe’s chairman, asked protesters not to return to the drilling site. Instead, he urged supporters to join in a planned march on Washington on 10 March.

“We ask that our allies join us in demanding that Congress demand a fair and accurate process,” Archambault said in a statement. “Our fight is no longer at the North Dakota site itself. Our fight is with Congress and the Trump administration. Meet us in Washington on 10 March.”

“We call on the Native Nations of the United States to stand together, unite and fight back,” he added. “Under this administration, all of our rights, everything that makes us who we are is at risk. Please respect our people and do not come to Standing Rock and instead exercise your first amendment rights and take this fight to your respective state capitols, to your members of Congress, and to Washington DC.”

Thousands of environmental activists and indigenous activists spent months living at several camps near the Standing Rock reservation and pipeline construction project, with demonstrations that frequently ended in violent clashes with police.

“I feel completely betrayed as an American citizen,” said Cheryl Angel, a member of the Sicangu Lakota tribe who spent months at Standing Rock. “I feel like the government has stabbed Americans in the back. Their decision to approve that pipeline without an environmental impact study, no one will know the detriment to that river until the pipeline leaks.”

Angel said she planned to return to Standing Rock as soon as possible and that she expected many others would show up to fight the project.

“I’m hoping for an avalanche of people – a downpour of water protectors.”

Kandi Mossett, a member of the Mandan, Hidatsa and Arikara tribes, said there would be demonstrations across the country to protest against Trump and to support the tribe’s efforts to stop the pipeline in court.

The activist with the Indigenous Environmental Network said she expected the company to start drilling as soon as possible.

“We know that the company has had workers on standby in Bismarck,” she said. “They’re probably going to try and come in immediately.”

Mossett, who lives in North Dakota, said the groups would keep fighting the oil industry no matter what happens at Standing Rock: “The Dakota Access pipeline is a symptom of the larger problem, which is the fracking that’s continuing to happen. Society as a whole needs to wake up and realize there are no jobs on a dead planet.”