ZURICH—The head of the Swiss National Bank reiterated concerns that a popular vote on requiring the central bank to keep a fifth of its assets in gold would hinder its ability to conduct monetary policy.

In text of a speech to be delivered Sunday, SNB Chairman Thomas Jordan said the initiative, known as “Save Our Swiss Gold,” would limit the central bank’s “room for maneuver.” That would make it harder for the bank to intervene during crises and fulfill its mandate of price stability.

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