Mr Papandreou, who served as prime minister from 1981 to 1989, said he had "never felt such a stranger within the EU" as at Cannes. Diplomats from other EU countries said he felt insulted at the way President Jacques Chirac of France, the summit host, had publicly criticised Greek policies on Tuesday at an end-of-summit news conference.
Mr Chirac, a centre-right Gaullist, laid into Mr Papandreou for breaking ranks with his EU allies and imposing a trade embargo on the Former Yugoslav Republic of Macedonia, a country that Greece refuses to recognise on the grounds that its name and flag imply a claim to Greek territory. Mr Chirac also annoyed Mr Papandreou by saying it was time for the EU to develop a closer relationship with Turkey, Greece's main rival.
Greece's foreign ministry spokesman, Constantine Bikas, responded to Mr Chirac's criticisms yesterday by deriding the French leader's understanding of Turkish politics. Referring to Turkey's Prime Minister, Tansu Ciller, Mr Bikas said: "We consider that the logic of unconditional support for Ciller so that [Islamic] fundamentalism does not prevail is simplistic and dangerous."
He continued: "We believe that the same kind of simplistic mistake was made in the 1930s. Humanity paid a heavy price for supporting Nazism in order to contain Bolshevism."
Even before the Cannes summit, Mr Chirac and Mr Papandreou had locked horns. At a dinner for EU leaders in Paris on 9 June, the Greek leader was talking about the right of Serbs to defend their Orthodox faith in former Yugoslavia when Mr Chirac interrupted: "Don't talk to me about wars of religion. These people have no faith and know no law. They are terrorists."
Stung by his treatment at Cannes, Mr Papandreou told reporters that the Greek people should prepare themselves for battle against "decisions taken by the triumvirate of big powers" - presumably France, Germany and Britain, all of which have centre-right governments. But sympathy for Mr Papandreou seems restrained even among his fellow socialist leaders in the EU.
Spain's Prime Minister, Felipe Gonzalez, a socialist who will replace Mr Chirac as head of the EU's presidency on 1 July, made clear he supported the French proposal to cultivate closer ties with Turkey. A scornful Mr Papandreou said: "Mr Gonzalez has identified himself with neo-liberalism. The only socialist thing left is the name of his party."