A federal judge on Monday blocked the proposed $2.2 billion merger of two major publishers, Penguin Random House and Simon & Schuster, delivering a significant win for the U.S. Justice Department’s antitrust division in its drive for more aggressive enforcement.

U.S. Circuit Judge Florence Pan of the District of Columbia said in a brief two-page order the Justice Department had shown that the merger of two so-called “Big Five” publishers would substantially lessen competition “in the market for the U.S. publishing rights to anticipated top-selling books.” Pan, who was recently confirmed to the U.S. Court of Appeals for the D.C. Circuit, stayed on the case to issue a final ruling as a district court judge.