OTTAWA - First Nations in British Columbia are warning the federal government that they will stop at nothing to prevent a mining company from destroying a fishing lake to process copper and gold.

"This lake, as I've indicated, is fundamental to the Tsilhqot'in people. The destruction of this lake is not an option," said regional chief Jody Wilson-Raybould, on behalf of the Assembly of First Nations.

"We are here sitting, with unity of purpose," warned Wilson-Raybould, surrounded by other regional and local First Nations representatives.

The federal cabinet is expected to decide by next Friday whether it's worth sacrificing the health of Fish Lake in northern B.C. and other aspects of the local environment for the jobs that the $800-million Prosperity mine project promises to bring.

The First Nations groups are in Ottawa to lobby key federal ministers and explain their case to the public.

A recent federal review panel found that the project would impose

"significant adverse environmental effects" on fish, fish habitat, grizzly bears and First Nations' use of the land for traditional purposes.

But the B.C. government approved the mine anyway -- putting pressure on Ottawa to agree and give the project a final green light.

If Ottawa breaks with tradition and chooses economics over environment, First Nations groups say they will do whatever it takes to prevent work from going ahead -- from lobbying and litigation to protest and even physical confrontation, if need be.

"Our people are willing and ready to defend our lands," said local Chief Marilyn Baptiste.

"As one of my elders had said when we were going through the panel hearings, she will be there on the road in her wheelchair. She will have her shotguns, and she will not miss."

She said her people fought off gold prospectors in the late 1800s and are prepared to do so again.

"We are willing to sacrifice our lives. I am willing to sacrifice my life for the sake of saving our lands and future generations," she said.

The local bands are backed by regional and national organizations as well as 12 environmental groups.

The B.C. government argues that Taseko Mines Ltd. should go ahead with the mine, partly because the economic benefits during the 20-year life of the mine would outweigh the environmental harm.

The provincial assessment predicted a $5-billion economic injection over the life of the mine, and $600 million in tax revenues for various governments.

The mine would be about 250 kilometres north of Vancouver in an area that has struggled to deal with the mountain pine beetle, the collapse of the forestry industry and low commodity prices.

Taseko says the mine is worth about $3 billion in today's terms.

The open-pit mine would use Fish Lake and Little Fish Lake as tailings ponds. The company would replace the destroyed lakes by an artificial lake, but the quantity of trout in that lake would be far, far lower than in the natural habitat.

While the federal environmental review does not specifically compel the federal cabinet to stop the Prosperity mine, Ottawa has traditionally respected the conclusions of environmental assessments, the First Nations groups argue.

"That report leaves the government legally, constitutionally and morally bound to reject this proposal," said Chief Perry Guichon of the Alexis Creek First Nation.

"Yet the province of B.C. and particularly the company proposing this mine, Taskeo Mines Ltd., have talked as if the federal approval was merely a formality and that approval would be quickly granted."

Indeed, the company's website states that it "fully expects final approval in September," and is planning to start major construction next spring.