The ECB had made it difficult for the banks to open on Monday because it decided to freeze the level of funding support it gives the banking system, rather than increasing it to cover a rise in withdrawals from worried depositors.

Amid drama in Greece, where a clear majority of people want to remain inside the euro, the next few days present a major challenge to the integrity of the 16-year-old Eurozone currency bloc. The consequences for markets and the wider financial system are unclear.

The head of Piraeus Bank, one of Greece’s top four banks, speaking after a meeting of the country’s financial stability council, said banks would be shut on Monday while a financial industry source said the Athens stock exchange would not open.

“It is a dark hour for Europe … nevertheless from where we’re sitting we have a clear conscience,” Greek Finance Minister Yanis Varoufakis said earlier in an interview with the BBC.

Asked whether Greece would close banks and impose capital controls he said: “This is a matter that we’ll have to work overnight on with the appropriate authorities both here in Greece and [with the ECB] in Frankfurt.”

Meanwhile, U.S. President Barack Obama and German Chancellor Angela Merkel on Sunday agreed that Greece needed to find its way back to a path of reform and avoid an exit from the Eurozone, the White House said. Late on Saturday, the Greek Parliament voted in favour of holding the referendum on July 5.

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