Speaking on RTL Radio, Juncker said Greek Prime Minister George Papandreou had taken the decision to propose a referendum without consulting other European leaders, a move that piles "great nervousness and insecurity" on top of an already insecure situation for the euro zone economy.

"The Greek prime minister has taken this decision without talking it through with his European colleagues," Juncker said.

"It is something that brings a great nervousness, that adds great insecurity to already great insecurity and therefore we need to see calmly how we will deal with this."

Asked whether, if Greeks were to vote 'no' in the referendum, it would mean bankruptcy for Greece, Juncker responded:

"I cannot exclude that this would be the case, but it depends on how exactly the question is formulated and on what exactly the Greeks people will vote on."