Pipeline company TransCanada is waiting for the U.S. State Department to sign off on plans for its Keystone XL pipeline.
EU Climate Commissioner Connie Hedegaard said in Washington that rejecting the project would show that the White House is serious about climate change issues.
"That would be a strong signal to the world," she was quoted by the Platts news service as saying.
Pipeline opponents say pipelines like Keystone XL, which rely on carbon-intensive tar sands oil from Alberta, are too great of an environmental threat. Hedegaard has expressed concern about the level of greenhouse gas emissions generated from oil sands production.
Project supporters, however, say it would help ensure North American energy security and provide some level of economic stimulus. TransCanada states that it believes "pipelines are (a) safe and environmentally favorable way" to transport crude oil.
Washington needs to sign off on the project because it would cross the U.S.-Canadian border.