Credit rating agency Fitch has Ireland on a list of countries that would face a downgrade in the event of Greece leaving the euro zone.

It has put the whole of the euro zone on notice that were Greece to leave the currency bloc, the remaining countries could find their sovereign ratings at risk.

The countries most at risk were, it said, France, Italy, Spain, Cyprus, Ireland, Portugal, Slovenia and Belgium

It said it was likely to put all euro area ratings on negative watch if Greece were to leave and that those countries which currently have a negative outlook on their ratings would be at most immediate risk of a downgrade.

"In the event of Greece leaving (the euro), either as a result of the current political crisis or at a later date as the economy fails to stabilise, Fitch would likely place the sovereign ratings of all the remaining euro area member states on Rating Watch Negative as it re-assessed the systemic and country-specific implications of a Greek exit," Fitch said in a statement.

The agency, whose decisions along with those of Moody's and Standard & Poor's help set the cost of borrowing by governments, said the extent of any downgrades would depend on how the euro zone reacted to Greece leaving.

"The probability and magnitude ... would largely depend on the European policy response and its success in limiting contagion, as well as outlining a credible vision of a reformed (euro zone)," it said.

"Nonetheless, the sovereign ratings of all euro zone member states would potentially be at risk," Fitch said.

Talks on forming a new government collapse

Greek socialist leader Evangelos Venizelos admitted on Friday evening that he had failed in a last-ditch bid to form a government after a key leftist party ruled out joining a pro-austerity coalition.

"I am going to inform the president of the republic tomorrow and I hope that during the meeting with Carolos Papoulias, each party will assume its responsibilities," said Venizelos, referring to talks aimed at trying to form an emergency unity government.

The majority of Greeks want to stay in the euro zone but voted last Sunday for parties that reject the severe terms of a bailout negotiated with foreign lenders.

European leaders say Greece will be ejected from the common currency if it turns its back on the package of tax hikes and wage cuts.