Switzerland's cantonal police chiefs are warning that a yes vote on the SVP's "No to 10 million" initiative would seriously compromise Swiss security.
Karin Kayser-Frutschi, who heads the Conference of Cantonal Police Directors, says approval at the 14 June referendum could end Swiss access to the EU's police database.
She warns the country could become "an isolated island within Europe" and a haven for organised crime, terrorism and human trafficking.
The umbrella group for Swiss police officers issued a similar warning in April, pointing to the loss of access to the Schengen Information System - Europe's largest security and border-management database.
Switzerland's cooperation agreement with Europol would also be at risk.
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