ISTANBUL—One of the prosecutors involved in a sprawling corruption investigation was removed Thursday after complaining that his efforts were being blocked, adding to Prime Minister Recep Tayyip Erdogan's woes as elections approach.

The embattled premier suffered another setback in his attempt to rein in the graft probe—which has targeted some close allies and already prompted a cabinet shuffle—when his move to curb prosecutorial independence was called unconstitutional.

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