The EU must insist that the U.K. force its overseas territories and crown dependencies to adopt transparency rules that require companies to disclose their owners as part of any future post-Brexit trade relationship.

John Christensen, director of the U.K.-based advocacy group Tax Justice Network (TJN), insisted that the European Union should only allow City of London financial service companies equivalent access—enjoyed by EU member states—to EU customers, if the U.K. overseas territories and crown dependencies adopt strict company ownership...