LONDON—The London Metal Exchange chalked up a big victory over investors on Wednesday, when a court said it was within its rights to cancel trades during a high-profile blowup in the nickel market sparked by Russia’s full-scale invasion of Ukraine.

The U.K. court rejected claims by activist fund Elliott Investment Management and market maker Jane Street Global Trading that the exchange had overstepped its powers by nixing $12 billion in trades in March last year. Had they won, Elliott and Jane Street planned to claim hundreds of millions of dollars in damages combined in a follow-on trial.