By Viktoria Dendrinou 

BRUSSELS--Greece has five days to strike a deal with its international creditors over a new bailout program before European Union leaders convene in Brussels on July 12, European Council President Donald Tusk said Tuesday.

Mr. Tusk, who presides over the leaders' summit said that the end of this week is the final deadline for a deal between Greece and its lenders on reforms the country must undertake in exchange for sustained financial aid.

The inability to find an agreement by then "may lead to the bankruptcy of Greece and the insolvency of its banking system," he said, speaking at a press conference following a summit of eurozone leaders in Brussels.

"I have no doubt, this is the most critical moment in our history, EU and eurozone," he added.

His comments come after eurozone leaders were called to Brussels urgently, following the results of a referendum called by Greek Prime Minister Alexis Tsipras in which voters overwhelmingly rejected demands by creditors on required cuts and economic overhauls.

Failure to strike a deal could catapult Greece outside the common currency, European officials have said.

"We have a Grexit scenario prepared in detail," said Jean-Claude Juncker, the president of the European Commission, the EU's executive arm.

Write to Viktoria Dendrinou at viktoria.dendrinou@wsj.com