In a decision made public on Tuesday, U.S. District Judge Katherine Forrest in Manhattan concluded that the LME was an "organ" of the UK government, and therefore immune from the lawsuit under the Foreign Sovereign Immunities Act.

Forrest acknowledged that her decision may at first glance seem "somewhat surprising and counterintuitive," noting that the LME is a privately-held, for-profit company subject to extensive regulation.

But she said the relevant case law "tips decidedly" toward a grant of immunity, noting that the LME is required by law to perform "the decidedly public function of market regulation."

The decision does not affect other defendants in the case, which include the large mining company Glencore Plc, Goldman Sachs Group Inc, JPMorgan Chase & Co, and various commodity trading, metals mining and metals warehousing companies.

It is nonetheless a setback for so-called direct purchaser plaintiffs, commercial end-user plaintiffs, consumer end-user plaintiffs and other entities pursuing the class-action case.

They accused the defendants of having conspired since May 2009 to drive up aluminium prices by reducing supply, resulting in delays, sometimes referred to as queues, of up to 16 months to fill orders.

The LME allegedly profited illegally from its share of the higher storage revenue collected by approved warehouses, plaintiffs have said.

Christopher Lovell, a partner at Lovell Stewart Halebian Jacobson representing some plaintiffs, said there will be an appeal.

"Although the opinion is reasoned, plaintiffs' position is that the court believed that it was constrained by precedent," he said.

The LME in a statement said it was pleased with the decision. "We continue to take action to manage queue-related issues in accordance with our obligations," it said.

Margaret Zwisler, a partner at Latham & Watkins representing the LME, had told Forrest during oral argument in June that the LME's main job under UK law was to maintain an orderly market in metals, including by consulting with market participants through a process similar to processes used by U.S. government agencies.

Forrest oversees 26 lawsuits that were combined before her last December. She said the plaintiffs cannot replead their case against the LME. Her decision is dated Aug. 25.

The case is In re: Aluminum Warehousing Antitrust Litigation, U.S. District Court, Southern District of New York, No. 13-md-02481.

(Reporting by Jonathan Stempel in New York; Additional reporting by Josephine Mason; Editing by Lisa Von Ahn and Tom Brown)

By Jonathan Stempel

Stocks treated in this article : JPMorgan Chase & Co., Goldman Sachs Group Inc, Glencore PLC