In the wide-ranging interview published in the El Mundo daily, Luis de Guindos warned that a government composed of the long-established Socialist party (PSOE) and upstart Podemos would generate a lot of "uncertainty."
Spain has been in political limbo since December elections saw the outgoing, conservative Popular Party (PP) win but without an absolute majority, leaving Prime Minister Mariano Rajoy struggling to form a government due to lack of support from other groupings.
Such a coalition "would be seen as an unstable government, as Podemos and the PSOE are competing for the same electoral space," De Guindos said in the interview.
When asked whether the markets were more afraid of anti-austerity Podemos or of the resurgent secessionist threat in Spain's northeastern region of Catalonia, he responded that an "unstable government" was more worrying.
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"There is concern that reforms will be reversed, that the public deficit won't be addressed, that competitivity will be lost," he was quoted as saying.
Spain has now started to recover, but the Socialists and Podemos argue this has been at the expense of social welfare as inequalities have risen drastically.
Anti-poverty agency Oxfam warned last week that out of the OECD group of most advanced or emerging economies, Spain was the nation where inequalities had risen the second-most -- far more than even crisis-hit Greece or Mexico.