SOFIA, Bulgaria—Bulgaria, the European Union's poorest member state and a rare fiscal bright spot for the bloc, has indefinitely frozen long-held plans to adopt the common currency, marking the latest fiscally prudent country to cool its enthusiasm for the embattled currency.

Speaking in interviews in Sofia, Prime Minister Boyko Borisov and Finance Minister Simeon Djankov said that the decision to shelve plans to join the currency area, a longtime strategic aim of successive governments in the former communist state, came...