* Army cancels environmental study on pipeline impact
* To grant permit to complete $3.8 bln line
* Comes after President Trump ordered speedy review
* Tribe plans legal challenge, Washington protest (Adds acting army secretary, more from Standing Rock Sioux, background)
By Valerie Volcovici and Ernest Scheyder
WASHINGTON/HOUSTON, Feb 7 (Reuters) - The U.S. Army will grant the final permit for the controversial Dakota Access oil pipeline after an order from President Donald Trump to expedite the project despite opposition from Native American tribes and climate activists.
In a court filing on Tuesday, the Army said that it would allow the final section of the line to tunnel under North Dakota's Lake Oahe, part of the Missouri River system. This could enable the $3.8 billion pipeline to begin operation as soon as June.
Energy Transfer Partners is building the 1,170-mile (1,885 km) line to help move crude from the shale oilfields of North Dakota to Illinois en route to the Gulf of Mexico, where many U.S. refineries are located.
Protests against the project last year drew drew thousands of people to the North Dakota plains including Native American tribes and environmental activists, and protest camps sprung up. The movement attracted high-profile political and celebrity supporters.
The permit was the last bureaucratic hurdle to the pipeline's completion, and Tuesday's decision drew praise from supporters of the project and outrage from activists, including promises of a legal challenge from the Standing Rock Sioux tribe.
"It's great to see this new administration following through on their promises and letting projects go forward to the benefit of American consumers and workers," said John Stoody, spokesman for the Association of Oil Pipe Lines.
The Standing Rock Sioux, which contends the pipeline would desecrate sacred sites and potentially pollute its water source,
vowed to shut pipeline operations down if construction is completed, without elaborating how it would do so. The tribe called on its supporters to protest in Washington on March 10 rather than return to North Dakota.
"As Native peoples, we have been knocked down again, but we will get back up," the tribe said in the statement. "We will rise above the greed and corruption that has plagued our peoples since first contact. We call on the Native Nations of the United States to stand together, unite and fight back."
Former President Barack Obama's administration last year delayed completion of the pipeline pending a review of tribal concerns and in December ordered an environmental study.