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Schengen (the passport-free travel zone) is one of the biggest achievements of the European integration process. Without Schengen, without the free movement of workers, without freedom of European citizens to travel, the euro makes no sense, and the same applies to the link between Schengen, freedom of movement and the internal market. If anybody wants to kill off Schengen, then ultimately what they are going to do is do away with the single market as well.