BERLIN—The leaders of France, Germany and Italy pledged to set new policy priorities for the European Union by September and implement projects to increase economic growth and security in a bid to prop up flagging popular support for the bloc after the U.K. voted to leave.

The EU leaders also said the bloc’s 27 other member states wouldn’t hold negotiations about the outlines of the U.K.’s future relation with the EU until London formally notified the bloc of its intention to leave.

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