The European Union’s decision to block a planned $17 billion tie-up between securities exchange operators NYSE Euronext and Deutsche Börse was legitimate, the EU’s second-highest court said Monday, rejecting an appeal from Deutsche Börse.

The deal, which would have created the world's largest combined market for trading stocks and derivatives, was rejected by EU regulators in early 2012. US antitrust authorities had approved the deal in late 2011, subject to relatively minor conditions.