The European Commission said it had not been notified formally by the Greek government of its plans to hold a referendum on a 130 billion euro ($180 billion) bailout.
Greek Prime Minister George Papandreou announced the vote late on Monday, threatening to worsen euro zone crisis and sparking complaints in Germany that Athens is trying to wriggle out of the deal.
"The thing that is holding us back formally is that the Greeks have formally not informed us (about the referendum)," said the European Commission's spokeswoman.
"Until we have, for example, a letter, or something like that, from the Greek prime minister, I wouldn't expect us to react." She said the European Commission was working on a statement to release and was waiting for Greece to confirm the vote.
Euro zone leaders agreed last week to hand Athens a second bailout and a 50-percent write-down on its debt. The price of the package is a program of harsh state spending cuts that have caused a tide of anger among Greeks.