(Rewrites throughout, adds detail on Pathways Alliance, Bill C-59 and comments from industry association)

OTTAWA, June 20 (Reuters) -

The Pathways Alliance, a group of Canada's biggest oil sands producers, has removed all content about environmental goals from its website and social media pages, citing "significant uncertainty" over the federal government's proposed anti-greenwashing legislation.

Bill C-59, which only needs royal assent to become law after Canada's parliament gave approval on Wednesday, would force companies to show proof of their environmental claims.

Pathways is proposing a C$16.5 billion ($12.04 billion)carbon capture and storage project to cut carbon emissions from the oil sands, but has faced criticism from environmentalists for slow progress and seeking more government financial support.

On Thursday, the content on Pathways' website had been replaced with a post about its concerns with omnibus Bill C-59, which will implement the Liberal government's mid-year budget proposals and change Canada's Competition Act.

"Imminent amendments to the Competition Act will create significant uncertainty for Canadian companies that want to communicate publicly about the work they are doing to improve their environmental performance," the website said.

On Wednesday, the website had said "Canada's oil sands are on a path to reach net-zero emissions from operations," according to an archived copy.

The alliance, which includes oil sands producers Suncor Energy, Imperial Oil, and Cenovus, did not immediately respond to a request for further comment.

The post on Pathways' website also said specific guidance from Canada's Competition Bureau may help direct its communications approach in future, and added that the group "remains committed to the work we are doing."

Canada is the world's fourth-largest oil producer and the oil and gas sector is the country's highest-polluting industry, accounting for more than a quarter of all emissions.

Climate advocacy group Environmental Defence said Pathways' decision to take down content shows "they don't have evidence to support the story they're selling."

The Canadian Association of Petroleum Producers (CAPP) said it was extremely disappointed with the amendments.

"The effect of this legislation is to silence the energy industry and those that support it in an effort to clear the field of debate and to promote the voices of those most opposed to Canada's energy industry," CAPP President Lisa Baiton said in a statement.

Bill C-59 was amended earlier this year to add anti-greenwashing rules and has already cleared the lower House of Commons and awaits passage by the unelected upper chamber of the parliament. ($1 = 1.3699 Canadian dollars) (Reporting by Nia Williams in British Columbia and Ismail Shakil in Ottawa Editing by Marguerita Choy)