A state law that makes it illegal to sell a popular Irish butter in Wisconsin is unconstitutional and deprives consumers of their rights, according to a lawsuit filed Thursday in Ozaukee County court.

The public advocacy group Wisconsin Institute for Law and Liberty brought the suit against the state on behalf of five clients, four of them consumers and one a Grafton food store.

"Wisconsin’s current protectionist law requires butter that is bought and sold to be labeled by the government. This archaic labeling regime prevents very popular butter such as Kerrygold from being enjoyed by Wisconsin residents," the group said in a news release.

A state law with roots in the 1953 margarine scare requires all butter sold in Wisconsin to be tested and graded by state-approved experts.

As a butter made and packaged in Ireland, Kerrygold is not inspected in the United States, making it illegal to sell under the state law.

Consumers started voicing their displeasure on social media in late 2015 when Kerrygold began vanishing from Wisconsin grocery stores.

That is when state inspectors began warning stores and distributors about the law.

That enforcement action followed a complaint that Kerrygold was being sold by some retailers in violation of the law.

The Wisconsin butter grading law is enforced by the state Department of Agriculture, Trade and Consumer Protection. A spokesperson for the agency would not comment on pending litigation.

The distributor of Kerrygold butter in the United States, Ornua Foods North America, also declined to comment on the lawsuit.

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