Barack Obama has rejected the building of a controversial oil pipeline between the US and Canada.

The Keystone XL was the proposed fourth phase of the existing Keystone Pipeline System, and would have linked Alberta and Nebraska.

Debate has raged over the proposed extension since 2008. Environmental activists argued the building of the pipeline would have led to greater potential for oil spills and increased carbon emissions, while proponents suggested it would have seen a significant boost in the economy for both countries.

Despite years of protests and campaigning against the pipeline, opinion polls had shown a majority support for the project.

Announcing his rejection of the plans earlier today, President Obama said, "the pipeline would not make a meaningful, long-term contribution to our economy. The pipeline would not lower gas prices for American consumers. In fact, gas prices have already been falling, steadily.

"Shipping crude oil into our country would not increase America's energy security. If we want to prevent the worst effects of climate change before it's too late, the time to act is now," he added.

Responding to the rejection, Russ Girling - CEO of TransCanada, the company behind the pipeline - said "today, misplaced symbolism was chosen over merit and science - rhetoric won out over reason. Today's decision deals a damaging blow to jobs, the economy and the environment on both sides of the border".

The move reinforces President Obama's commitment to fighting climate change.

In August, he announced the revised Clean Power Plan, which will demand carbon emissions be cut by 32% from 2005 levels by 2030.

It will also encourage a shift towards renewable energy sources, including wind and solar energy.