Federal Liberals say the Harper government is in denial about the negative impact of oilsands development on Alberta's freshwater supplies.

They're calling on the government to fulfil its constitutional and legal obligations to protect fisheries and the health of aboriginal communities who live downstream from the massive energy projects in northern Alberta.

The Liberals' plea for urgent action comes just as U.S.-based environmental groups are expanding a controversial anti-oilsands ad campaign to Britain.

Billboard ads urge Britons to "think again" about visiting Alberta and feature grisly pictures of oil-soaked ducks found dead in an oilsands tailing pond.

The House of Commons environment committee launched a study into the impact of the oilsands on freshwater over two years ago. The four parties were unable to reach a consensus on the issue and will each issue separate reports.

The Liberal report accuses the government of effectively abdicating its responsibility to protect the quality of the water in the Athabasca River basin and the health of downstream native communities.

"In the final analysis, the story of the oilsands' relationship to water is very much a tale of denial by interested parties — private sector and governmental — of the potential negative consequences the industry might be having on a vital Canadian resource," the report concludes.

The report says toxic seepage from the tailing ponds and surface water run-off may be contaminating the Athabasca River.

Moreover, the oilsands extraction process requires vast amounts of water drawn from the river, potentially lowering water levels and harming fish and fish habitat.

Liberals are recommending that the government conduct a study of cancer rates in Fort Chipewyan, a downstream aboriginal community.

They also want the government to conduct year-round monitoring of the river's water quality, overseen by independent experts. And they want the government to fund research into the impact of the oilsands on fish, wetlands and water levels.