Imperial Oil plans to fight a government decision to revoke its water permit for the proposed $8-billion Kearl oilsands mine, a spokesman for the Calgary-based integrated oil and gas company said Monday.

"Obviously we're disappointed in the recent developments, but we do feel that the authorization that we have is legally valid," Gordon Wong said in an interview.

Imperial Oil three-month chart

Imperial has been granted an expedited Federal Court hearing for May 7, in which the company will seek to overturn the decision by the Department of Fisheries and Oceans.

In the meantime, work at the Kearl site will continue, Wong said.

"We will eventually need this permit, but the work that's underway right now at the site, and which is continuing, is not dependent on this particular permit."

The mine had been scheduled to start producing 100,000 barrels of bitumen a day in 2011, eventually ramping up to 300,000 barrels a day. Imperial's board of directors is expected to make a final decision about whether to build the mine by the end of September.

Wong said it is not clear how long the project's start-up would be delayed by the DFO move.

"We'll have a better sense of timelines once we hear back from the Federal Court," he said.

The court ruled in early March that there was not enough information on Kearl's greenhouse gas emissions and sent it back to a regulatory panel for another look.

Lawyers for environmental group EcoJustice have been arguing against the project, which would eventually strip-mine about 200 square kilometres of boreal forest and wetlands north of Fort McMurray.