Enbridge containment boom downstream of Battle Creek, Mich. (CBC)Enbridge containment boom downstream of Battle Creek, Mich. (CBC)

A public interest law firm is preparing to sue Calgary-based Enbridge Inc., owners of a pipeline that ruptured in southern Michigan and dumped millions of litres of oil into a Kalamazoo River tributary.

The Great Lakes Law Center on Monday sent Enbridge a notice of intent to file suit if a settlement isn't reached within 60 days.

The letter accuses the company of violating the U.S. Clean Water Act.

The Detroit-based law centre says Enbridge could face more than $26 million US in civil penalties based on the U.S. Environmental Protection Agency's estimate that the spill exceeds 3.8 million litres.

The company estimates the total at 3.1 million litres.

A phone message seeking comment was left with Enbridge, which reported the spill a week ago.

Enbridge shares rose 1.8 per cent to close at $41.51 US on the New York Stock Exchange Monday afternoon. The Toronto Stock Exchange was closed for the Canadian civic holiday.