European Commission President Jean-Claude Juncker said in Luxembourg on Friday that the Greek government's position will be "dramatically" weakened in negotiations with creditors if the Greeks vote "No" in Sunday's referendum.
Juncker made the remarks at a press conference marking the beginning of Luxembourg's six-month presidency of the Council of the European Union, Xinhua news agency reported.
"If Greeks vote 'No', the Greek position will be dramatically weakened," he said in a short response to a journalist's question.
"The programme has come to an end, there are no negotiations under way, if the Greeks vote 'No', they have done everything but strengthening the Greek negotiation position," he said.
"Even in the case of a 'Yes' vote, we will have to face difficult negotiations," he said.
Greek Prime Minister Alexis Tsipras has urged a "No" vote from citizens, saying it would enhance its hand in debt talks with creditors.
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"That is exactly not the case, I agree with President Juncker," said Luxembourg Prime Minister Xavier Bettel.
He said the Greek people "need to know what they are voting for".
"Voting a yes or no is not about whether Tsipras should stay or not, it is about whether or not Greece should stay in the eurozone," Bettel added. "It is about the future of the euro, it is about the future of the European Union."
Greece is set to hold a referendum on Sunday on a deal proposed by creditors.
According to the results of an opinion poll carried out by polling firm ALCO for Ethnos (Nation) newspaper, the "Yes" camp is slightly ahead with 44.8 percent against 43.3 percent for the "No" camp.