Environmental groups have filed notice that they plan to sue the owners of Montana's Colstrip power plant over alleged violations of federal pollution law.

The Sierra Club and Montana Environmental Information Center say the 2,200-megawatt plant has not upgraded pollution control equipment as required under the Clean Air Act.

They say such upgrades are required when substantial alterations are made to older plants like Colstrip.

A 60-day notice of the group's intent to sue in federal court was sent Tuesday to the plant's co-owners. Those are PPL Montana, NorthWestern Energy, Puget Sound Energy, Portland General Electric, Avista and PacifiCorp.

Colstrip is the second largest coal-fired power plant west of the Mississippi River.

PPL Montana spokesman David Hoffman says the suit notice stems from a coordinated attack against coal by environmentalists.