The Greek government made “completely unrealistic promises” to voters that it cannot now fulfil, the former European Commission president has said.
Jose Manuel Barroso, who left the key post last October, said that Alexis Tsipras’s Syriza-led administration lacked experience.
Greece’s demands were “completely unacceptable to other countries”, he told the BBC’s Business Live programme.
“It is not helpful if Greece attacks countries that are trying to help it.”
Mr Barroso added: “We should remember that there are poorer countries that are lending money to Greece, so to propose a cut to their debt would be certain to receive a no from their partners.”
He called on Greece to take responsibility for its financial plight and implement structural reforms, which was now the most important issue for the country.
“It was not Germany or any other member of the EU that created the problems in Greece, the problems in Greece are structural: low productivity and previous governments.”
Nations such as Ireland, Portugal and Spain had come back from the financial brink and Mr Barroso said Greece could do the same: “There is nothing regarding Greece that prevents it being successful, but… bad politics have created a lot of problems for Greece.”
Athens is still working on a list of economic reforms that will satisfy its international creditors – the European Central Bank, International Monetary Fund and the European Union.
Agreement on the reforms is needed to release further bailout cash that Greece needs to meet payments due later this month.
A government official said on Wednesday that Greece had sent an updated list to lenders in the hope of satisfying their demands for more detail.
‘Breaks a taboo’
Mr Barroso said a Greek departure from the eurozone would still be “negative”, but believed it would be less damaging now because financial markets were much more confident than they had been in recent years.
However, he added that a “Grexit” would still leave the idea of monetary union in doubt: “It breaks a taboo and sets a precedent.”
The former EC chief and Portuguese prime minister reiterated the idea that Britons would be worse off if the UK left the European Union.
“I believe they would lose a lot if it leaves the EU, because today, in the 21st century, versus the US, China, countries of 60 million people, they cannot speak at the same level – they do not have the same leverage – and we have to use the EU and our common leverage to count in the world.”
Mr Barroso, 59, is now a visiting professor at Princeton University and the University of Geneva.
Business Live airs at 07.30GMT each weekday on BBC World TV.