For the first time, a former U.S. president has come out against the Keystone XL pipeline.

The ex-president in question is Jimmy Carter.

The 39th president joined a group of Nobel laureates to sign a letter urging the current commander-in-chief to reject the pipeline from Canada.

The letter tells Barack Obama that he stands on the brink of making a choice that will define his legacy on one of the greatest challenges humanity has ever faced — climate change.

“History will reflect on this moment and it will be clear to our children and grandchildren if you made the right choice.... We urge you to reject the Keystone XL tar sands pipeline," the letter reads.

It says his decision will either signal a "dangerous commitment" to the status quo, or "bold leadership" that will inspire millions counting on him to do the right thing for the climate.

Obama has signalled that a decision on the Alberta-to-Texas pipeline is imminent before summer.

The federal and Alberta governments have campaigned heavily in Washington to advance the case for Keystone XL.

A spokesman for the Prime Minister's Office issued a statement in response to Carter's objections to the pipeline, pointing to the jobs it could create.

It also recalled U.S. difficulties with reliance on Middle Eastern oil during the Iranian revolution, when oil prices spiked on anticipation of supply shortages in the U.S.

"Our government’s position on the Keystone XL project is clear: the project will create tens of thousands of jobs for workers on both sides of the border, which will create tremendous economic benefits for both countries," the statement said.

"In addition, Mr. Carter knows from his time as president during the 1979 energy crisis, there are benefits to having access to oil from stable, secure partners like Canada. It’s also important to note that the U.S. State Department has more than once concluded that the Keystone project can be developed in an environmentally responsible manner, and we look forward to a positive decision as soon as possible."