U.S. President Barack Obama has vetoed a bill that would have approved construction of the Keystone XL oil pipeline.

The move, while expected, is still significant because it is only the third time that the current U.S. president has opted to shoot down a bill passed by Congress.

"I am returning herewith without my approval … the Keystone XL Pipeline Approval Act," the president said in a letter to the U.S. Senate notifying it of the veto. "Through this bill, the United States Congress attempts to circumvent long-standing and proven processes for determining whether or not building and operating a cross-border pipeline serves the national interest."

The White House position is that cross-border pipeline permits are a matter for the president to decide — not Congress — and that the Keystone XL bill was an attempt to usurp a presidential responsibility.

But the move doesn't signal the end for the 1,900-kilometre pipeline that would bring 800,000 barrels of Alberta crude oil every day to refineries on the U.S. Gulf Coast. The regulatory process is in its final phase as the State Department has finished collecting input and is now preparing a recommendation to the president. Obama must then decide whether the project is in the U.S. national interest.

President has 'open mind'

White House press secretary Josh Earnest said in a press briefing this morning that the president has an "open mind" on the Keystone XL pipeline and could very well approve it after the State Department review.

"They're going to evaluate the impact that this project would have on the country. They're going to have the opportunity to evaluate the impact this project would have on contributing to climate change," Earnest said.

"It certainly is possible," Earnest said in answer to a question about whether Obama would consider signing off on Keystone.

"The president will keep an open mind as the State Department considers the wide range of impacts that this pipeline could have on the country, both positive and negative," he added.

A presidential veto can be overridden if a bill gets two-thirds majority in both chambers of Congress — the Senate and the House of Representatives.

Senate majority leader Mitch McConnell indicated the Senate would start the process to try and override the veto by March 3. But it's unlikely Republicans can get enough support in both chambers to do that.

We are not going to give up in our efforts to get this pipeline built — not even close. — House Speaker John Boehner

"We are not going to give up in our efforts to get this pipeline built — not even close," Speaker of the House of Representatives John Boehner said. "We pledged to make the people's priorities our priorities, and we will keep working every day to deliver on that commitment."

Environmental groups and landowners from along the route have applauded the veto and urged Obama to go further and reject the project entirely.

For its part, TransCanada — the Calgary-based company that wants to build and operate the pipeline — was quick to say it stands by the project, and looks forward to working with legislators to see it completed.

Alberta premier 'disappointed'

"As we have done throughout the permitting process, TransCanada will keep working in good faith with the U.S. Department of State and other federal agencies to address any outstanding concerns with regard to Keystone XL," CEO Russ Girling said.

"Keystone XL is in the national interest of the United States and should be approved and constructed."

Alberta Premier Jim Prentice said he was "disappointed but not surprised" by the veto.

"While today’s decision was expected, it does not change the fact that Keystone XL would advance North American energy security and prosperity while offering the U.S. access to responsibly developed energy from a close ally and friend," he said in a press statement.

"Our commitment to responsible energy development is steadfast, and our environmental standards are much greater than those of other countries that send their oil to the U.S. market every day," Prentice continued.

Prentice said the debate is about whether Canadian oil will move by rail or by pipeline.

"The president … prefers to see it move by rail. We don’t think that’s the best choice."

Liberal environmental critic Geoff Regan said he believes the veto could have been avoided if the Harper government had put in place an environmental plan limiting emissions from oil development.

"The veto is a direct result of his refusal to adopt strong, credible environmental policies," Regan said.