SYRIZA leader Alexis Tsipras is threatening to tear up the punishing terms of the 130-billion-euro bailout that is keeping Greece from bankruptcy.
"If the radical left wins - which cannot be ruled out - the consequences for the currency union are unforeseeable,» Juncker, head of the group of eurozone finance ministers, told Austrian paper Kurier on Saturday.
"We will have to speak to any government. I can only warn everyone against leaving the currency union. The internal cohesion of the eurozone would be in danger."
In addition to the economic and social consequences for Greece itself, an exit would damage the entire currency union, he said, adding: «This has to be avoided. This would send a devastating signal. The Greeks must be aware of this."
Sunday's vote is a re-run of a May 6 election that produced a stalemate. No matter who wins the repeat vote, Juncker said, European leaders would have to quickly address Greece's reform program with the country's new leadership. «A third election would not be a solution."
He made clear, however, that renegotiating the substance of the conditions for international aid to Athens was out of the question.
Juncker said he would spend Sunday evening in telephone contact with European Central Bank President Mario Draghi and EU Economic and Monetary Affairs Commissioner Olli Rehn.