Environmental pressure group Greenpeace accused pipeline firm TransCanada of "dirty tricks" Tuesday, publishing leaked documents it said showed it plans to smear opponents of one of its projects.

TransCanada, the company behind the controversial Keystone XL pipeline to take oil to the United States, also plans to connect Alberta's oil sands to refineries and ports in eastern Canada.

It hired Edelman, one of the world's largest public relations firms, for strategic advice on the project and got back recommendations which "borrowed a page from the modern political playbook," according to Greenpeace.

The plan, written between May and August, warns TransCanada that winning public support is as important as winning regulatory approval, given Canadians' low level of trust in government.

In the plan, Edelman accuses "well-funded" environmentalist opponents of using opposition to specific pipeline projects as "proxies" for a broader debate about climate change.

To win approval for its Canadian Energy East project, Edelman urges TransCanada to try to discredit critics by questioning their motives and by providing discreet advice to opponents.

This could be achieved by recruiting third parties -- including landowners, unions, aboriginal tribes, businessmen and "credible community voices" -- to lobby on the pipeline's behalf.

These third party supporters should "put the pressure on, especially when TransCanada can't."

"Add layers of difficulty for our opponents, distracting them from their mission and causing them to redirect their resources," one of the leaked documents recommends.

Supporters should pressure politicians and regulatory agencies for project approval, and author op-ed pieces, blog posts or letters to newspapers to build public support for the project.

Greenpeace's Keith Stewart said: "These documents show that TransCanada is planning a secret dirty tricks campaign, using third parties to attack and smear its critics."

"When oil companies create front groups to smear their opponents, they degrade democratic debate.

"Greenpeace may be fierce in the defense of our environment, but we put our names to our statements and publish the research to back them up. We expect oil companies to do the same."

TransCanada spokesman Shawn Howard told media the company has simply provided Canadians with "the facts to make an informed decision about Energy East.

"Part of that includes ensuring that we understand what organized opponents are saying about our project," he said, adding that not all of Edelman's recommendations had been followed.