Athens Chamber of Commerce and Industry President Constantine Michalos is cautiously optimistic there will be a strong "yes" vote on Greece's referendum Sunday.
However, even if voters accept the euro zone's bailout terms, banks in the cash-strapped country likely won't open until at least July 20, he predicted.
"It's absolutely impossible [to reopen] unless there is a deal. A deal that passes all the parliaments of the euro zone, a deal that is struck here in Greece, signed and sealed, and then the ECB has to send funds," Michalos said in an interview with CNBC's "Power Lunch" on Thursday.
Those funds are needed because banking system funds do not exceed 600 million euros, he added.
"When banks do open, the credibility factor has to be at an acceptable level so that we don't have exactly the same phenomenon of Greeks' rushing to the banks to withdraw the remaining deposits."
Local banks and the Greek stock exchange remained shut for the fourth day in a row Thursday, and capital controls remained in place.
Greek State Minister Nikos Pappas told Reuters banks will reopen as soon as there is an agreement.
Greece defaulted on its 1.5 billion euro ($1.7 billion) payment to the International Monetary Fund on Tuesday, the same day its bailout program expired. Talks between Greece and its creditors are now on hold until after the referendum.
Prime Minster Alexis Tspiras is calling for voters to reject what he calls the "blackmail" of the euro zone and International Monetary Fund creditors.
However, Michalos said he believes the "yes" vote will win because Greek citizens view the referendum as a vote on euro and a majority of them are in favor of it.
—Reuters and CNBC's Evelyn Cheng contributed to this report.