By Mike Scarcella

July 30 (Reuters) - International Paper IP.N and other rival manufacturers have been accused in a new lawsuit in U.S. federal court of conspiring to fix and inflate prices for key raw materials that are used to make cardboard boxes and merchandise displays.

The proposed class action filed on Tuesday in the federal court in Chicago said International Paper, Packaging Corporation of America PKG.N, Georgia-Pacific and several other companies collectively raised prices by 30% since late 2020.

Mt. Vernon, New York-based Artuso Pastry Foods Corp filed the lawsuit on behalf of what it said was at least hundreds of thousands of buyers. The lawsuit said containerboard raw materials are used in pizza boxes, moving boxes, banker boxes and more.

The lawsuit said the manufacturers violated U.S. antitrust law through “numerous unprecedented and unjustified price increases, often implemented at the exact same time and for the exact same increase.”

International Paper declined to comment. Georgia-Pacific and Packaging Corporation of America did not immediately respond to requests for comment, and neither did defendants Smurfit Westrock SWR.L and Greif GEF.N.

Graphic Packaging GPK.N, also a defendant, in a statement said it was "confident in our practices and committed to operating with integrity."

Attorneys for the plaintiffs did not immediately respond to a request for comment.

The lawsuit said the defendants inflated prices for containerboard and linerboard, the materials that make up finished products such as cardboard boxes. The companies also make and sell boxes and other containerboard products, according to the lawsuit.

Price increases in the corrugated box market “drastically outpaced” what the lawsuit described as a modest rise in demand over the period covered by the complaint.

As demand fell in recent years, the lawsuit said, price increases continued.

The lawsuit said that in a competitive market, companies that raise prices substantially would not expect rivals to do the same thing.

The plaintiffs are seeking more than $5 million in alleged damages and a court order prohibiting any continued violations of antitrust law.

The case is Artuso Pastry Foods Corp v. Packaging Corporation of America et al, U.S. District Court for the Northern District of Illinois, No. 1:25-cv-08856.

For plaintiffs: Vincent Briganti of Lowey Dannenberg; Christopher Burke of Burke LLP; Eric Unrein of Cavanaugh, Biggs & Lemon; and Gary McCallister of McCallister Law Group.

For defendants: No appearances yet

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(Reporting by Mike Scarcella)