TransCanada has asked the US to suspend its review of the proposed Keystone XL pipeline in a move that might delay a decision until the US presidential election.
Environmentalists rushed to urge US President Barack Obama to refuse the company's request and instead end once and for all the project to carry oil from Canada through the US to the Gulf of Mexico.
The company's request for a pause, written to US Secretary of State John Kerry, points to a Nebraska regulatory process that could take another seven to 12 months.
Obama's successor, set to be elected on November 8, 2016, could then end up deciding the pipeline's fate.
A pause "will allow a decision on the permit to be made later based on certainty with respect to the route of the pipeline," TransCanada executive vice president Kristine Delkus wrote to Kerry.
The request for a suspension comes two days before Justin Trudeau of the Liberal party takes power as Prime Minister.
While not opposed outright to Keystone XL, he has expressed more reservations about the potential environmental impact than his predecessor Steven Harper.
Harper had been urging the United States to approve the 1900-kilometre pipeline to transport crude from Alberta to the Gulf of Mexico.
The Obama administration and the US environmental lobby have concerns about the potential impact of such a project, which would carry dense oil from the "tar sands" of central-western Canada, diluted with benzene.
Environmentalists urged Obama to reject TransCanada's request.
"This is nothing more than another desperate and cynical attempt by TransCanada to build their dirty pipeline someday if they get a climate denier in the White House in 2017," League of Conservation Voters senior vice president Tiernan Sittenfeld said in a statement.
"President Obama and Secretary Kerry have all the information they need to reject this dangerous pipeline, and we are counting on them to do just that."
Kerry recently said Trudeau's election victory won't influence any eventual US decision on the pipeline.
His predecessor at the State Department, Hillary Clinton, has come out against Keystone as part of her campaign for the White House, reinforcing Canadian fears that the project will be blocked.
But the Obama administration has yet to make a decision and parts of the energy sector north and south of the border are lobbying Washington to approve it.