Speaking at a news conference in Luxembourg to mark that country's accession to the presidency of the European Union, Juncker was blunt and to-the-point when asked to respond to Greek finance minister Yanis Varoufakis' contention that a 'No' vote would make the Greek negotiating position stronger.
The Greeks are being asked whether or not they will accept the latest proposals for reforms and austerity measures put forward by its creditors, the International Monetary Fund, the European Central Bank and the Eurozone countries.
Why we recommend a NO for the referendum — in 6 short bullet points http://t.co/7OtILGeSvz— Yanis Varoufakis (@yanisvaroufakis) July 1, 2015
Juncker said: "As the Greek referendum takes place on Sunday, I don't have the intention to be too long in my explanation. If the Greeks will vote no, the Greek position is dramatically weakened," at which point he turned off his microphone.
In a follow-up question about the Greek offer of more terms and conditions for the negotiations, he bluntly said:
"The program has come to an end. There are no negotiations on the way. If the Greeks will vote no they will have done everything to dramatically weaken the Greek position."
Last week Juncker launched a blistering attack on his critics and a barely veiled swipe at Alexis Tsipras over the breakdown in talks, which have left Greece on the edge of financial collapse. "We do not deserve this criticism," he said.
He said the talks had broken down in acrimony as: "Good will was thrown to the wind amid egotism, tactical games, populist games took precedent over other aspects.
"After all my efforts, I feel a little betrayed because not enough consideration has been given to my personal efforts and the efforts of others and made a sustained effort. There have been many rumours and that's really blocked out the voice of those who've worked tirelessly day and night.
"They have not spared any effort to keep the European family united. It's in the interests of the Greeks in particular. We hear reference to an ultimatum, take-it-or-leave it agreement. We've heard reference to blackmail. Where are these insults and threats coming from?"