The fund providing Greece's financial lifeline declared "an event of default by Greece".
The European Financial Stability Facility added, though, that it had decided to not immediately demand repayment of its loans - a step that analysts say could have triggered sudden "Grexit", or Greece's exit from the eurozone.
The news will come as a fresh shock to Greece's 11 million people, and will hang over two major, rival rallies taking place in Athens today seeking to galvanise 'Yes' and 'No' support for Sunday's referendum.
But Greek Prime Minister Alexis Tsipras rejects that, insisting a 'No' result would strengthen his hand and force international creditors withholding bailout funds to drop "humiliating" austerity terms.
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Only a last-minute challenge to the legality of the ballot in Greece's top administrative court, the Council of State, might be able derail it. The court is to give its ruling today.
Confusion, however, is widespread over the very technical question posed in the referendum.
That, and capital controls that have reduced Greeks to lining up at ATMs to make daily withdrawals capped at 60 euros (USD 67), has prompted many who formerly supported the government to swap sides.
An Alco institute poll found 44.8 per cent of Greeks intend to vote 'Yes' and 43.4 per cent are for 'No'. A Bloomberg survey for Greece's Macedonia University was equally split, showing 43 per cent to vote 'No' and 42.5 per cent 'Yes'.
European Commission chief Jean-Claude Juncker warned that Greece's negotiating position with creditors would be "dramatically weakened" in the event of a 'No'.
Even if the 'Yes' vote wins, there would still be "difficult" negotiations ahead, he added.
Greek voters, however, are confronted with a referendum question that has stumped many.