When the New York Times reported last year on warehousing practices that drive up the cost of aluminum, plaintiffs lawyers might have thought a winning case fell into their laps. But the resulting litigation lost a bit of its firepower this week, thanks to a daring and novel argument by Latham & Watkins on behalf of the London Metal Exchange Ltd.

In a 31-page ruling issued Monday, U.S. District Judge Katherine Forrest in Manhattan ruled that LME is an “organ” of the U.K. government, and therefore is shielded by sovereign immunity from claims that it conspired with Goldman Sachs & Co., JPMorgan Chase & Co. and others to restrain aluminum supplies and drive up prices. Forrest acknowledged that her conclusion might seem “surprising and counterintuitive” because LME is a privately held for-profit company.