Moody's slashed Greece's credit rating by three notches on Monday, raising the spectre that the distressed euro zone sovereign may be forced to restructure its debt, perhaps even before 2013.
The move increased pressure on European leaders to ease repayment terms on bailout loans to Greece, just as Germany and its allies appear to have turned their backs on radical steps to help Athens reduce its debt through bond purchases or buy-backs.
Moody's Investors Service downgraded Greek debt to B1 from Ba1 and said it may cut further, citing significant risks to the government's fiscal consolidation programme from a revenue shortfall and difficulties in reforming healthcare and state-owned companies.
"The sheer magnitude of the task becomes ever more apparent," Sarah Carlson, Moody's lead analyst on Greece, said.
The downgrade sent a ripple of concern around credit markets, raising the price of insuring Greek, Portuguese and Spanish debt against default and the risk premium on holding Greek bonds rather than benchmark German bunds.
Greece signed a 110 billion euros ($154 billion) rescue package with the EU and IMF last May to avoid default in exchange for draconian austerity measures which it has begun to implement. But many see the repayment terms as too onerous.
Even if it fulfils the entire three-year adjustment programme, its debt is projected to reach 158% of gross domestic product in 2013, a level widely seen as unsustainable.
"There is a risk that conditions attached to any kind of continuing support after 2013 could take solvency criteria into account that the country may not be able to satisfy, and therefore could result in a restructuring of existing debt," Carlson told Reuters.
Moody's was the first of the three major ratings agencies to classify Greek debt as "highly speculative", rating it lower than Egypt, and drew an indignant protest from Athens.
The Greek Finance Ministry said Moody's had ignored progress in implementing its fiscal consolidation plan, including an improvement in revenue collection.
"The rating downgrade announced by Moody's today is completely unjustified," it said in a statement.
But it did say the downgrade would add to Greece's problems.
"Decisions such as Moody's today can initiate damaging self-fulfilling prophecies," it said.
However, some analysts said the bond market was already pricing in a managed Greek default.
"This is not going to be the last downgrade for Greece," said Christoph Weil, an economist at Commerzbank. "The market has already discounted that Greece will need to restructure its debt, so the rating agencies are just running behind the market."
Euro zone leaders will hold summit talks on Friday to discuss measures to enforce stricter budget discipline, boost economic competitiveness and strengthen the bloc's financial rescue fund in an attempt to draw a line under the debt crisis.
European Monetary Affairs Commissioner Olli Rehn told a German newspaper on Saturday that countries that share the euro currency must grant Greece and fellow debt-strapped nation Ireland easier terms on loans they have taken.
Germany, the EU's reluctant paymaster, has hinted it may agree to extending the maturity of Greek loans to seven years, like Ireland's, and possibly ease the interest rate slightly.
But Berlin's ruling centre-right coalition parties and the Bundesbank have strongly opposed any purchase of distressed sovereign bonds by the euro zone rescue fund and any lending to Greece to buy back its own debt on the market at a discount.
Moody's said it was concerned by the lack of certainty about the nature of financial support that will be available to Greece after 2013, and its implications for bondholders.
"The likelihood of a default or distressed exchange has risen since its last downgrade of the Greek government debt rating in June 2010," the agency said.
However, Carlson stressed: "Our central case remains that the Greek government is going to achieve its objectives without needing to impose losses on creditors."
Private economists are less sure.
"Greek debt remains high and there is always the risk of a haircut," said Juergen Michels at Citigroup. "We expect, not immediately but in the coming years, that more measures will be needed, maybe even a haircut."
The spread on 10-year Greek debt against benchmark Bunds widened by 8 basis points following the Moody's downgrade.
The latest ratings action came amid intense negotiations among euro zone countries on a package of measures intended to overcome the sovereign debt crisis that has shaken the single currency area since November 2009.
Earlier this month, Standard & Poor's said it may cut its rating of Greece, depending on the details of Europe's crisis fund that euro zone policymakers are discussing.
Centre-right leaders meeting in Helsinki last Friday agreed in principle to increase the effective lending capacity of the temporary European Financial Stability Facility and review the loan conditions to Greece and Ireland.
But Germany, Finland and the Netherlands opposed allowing the rescue fund to buy bonds or to lend distressed countries money to buy their own bonds, participants said.
Some EU officials see the hardline stance as pushing Greece towards restructuring, but only after west European banks have had time to raise their capital base to cope with the fallout from potential Greek losses.