T-Mobile US Inc. and Sprint Corp. are waiting for a federal judge to rule on whether they can merge, but the companies face another hurdle even if they overcome that legal challenge: the California Public Utilities Commission.

The state utilities overseer is the only such body that hasn’t yet blessed the $26 billion deal, and its continuing review threatens to further delay—or even derail—a merger that has dragged on for nearly two years. The state body has until July to vote but might extend that timeline further.

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