European Commission President Jean-Claude Juncker warned Britain Tuesday that voting to leave the European Union (EU) would be "an act of self-harm" that would endanger everything Europeans had worked together to achieve.

"To turn your backs on your neighbours and retreat into isolation would go against everything that Europe and the UK stand for," Juncker said during an address to Greek business leaders in Athens, adding: "To leave would be an act of self-harm."

"All too often we take for granted what we have built," Juncker said, listing peace, freedom, prosperity and a "way of life that is the envy of the entire world" among the EU's achievements.

"This is the patient work of generations of women and men, and we could not have done it without the British people," he added in an impassioned appeal for Britons to remain within the 28-member bloc.

"Leaving the EU would put all this at risk. It would damage what we have achieved so far. And it would diminish what we can achieve tomorrow," Juncker declared, insisting: "Europe is stronger together."

Juncker had also spoken about Brexit with the German press where he sounded a positive note about the vote.

"I hope that the British will let themselves be guided by their pragmatism which is a British virtue," he said in an interview with the Frankfurter Allgemaine Zeitung to be published Wednesday.

Meanwhile Greek Prime Minister Alexis Tsipras, who spoke before Juncker, said that Brexit would "plunge the EU into turmoil".

He said that regardless of the referendum's outcome, "We must admit that Europe is going through a political crisis," blaming the turbulence in the bloc on the EU's "austerity recipes".

Last month, Juncker warned that British "deserters will not be welcomed with open arms" by European partners if Britain voted to leave.