Moody's credit rating agency downgraded Greek debt by three notches on Monday and warned that the eurozone rescue was almost certain to trigger another two-notch cut to default status.

Moody's, taking a similar line to the Fitch agency on Friday, said that once old debt had been replaced with new bonds on easier terms under the rescue scheme, it would assess the new instruments and issue a new notation.

A default rating could have unforeseeable domino effects on financial markets, but the ISDA organisation which oversees CDS default insurance contracts said the rescue terms would probably not trigger payout clauses.

Averting a default, and triggering CDS turmoil, was a key obstacle in the rescue talks, but eventually eurozone governments resigned themselves to this possibility.

Moody's Investors Service said that the second rescue announced on Thursday meant that private sector holders of Greek bonds "are now virtually certain to incur credit losses."

This rescue for Greece involves initially up to 2014 about 110 billion euros from eurozone governments in various forms and 50 billion euros from banks. But Moody's said the effect would be "limited."

The agency, which issued two statements on the rescue, said that it had "downgraded Greece's local- and foreign-currency bond ratings to Ca from Caa1 and has assigned a developing outlook to the ratings".

This reflected "the current uncertainty about the exact market value of the securities creditors will receive in the exchange."

It explained that "if and when the debt exchanges occur, Moody's would define this as a default by the Greek government on its public debt."

The rescue offered short-term relief both to Greece, and to the eurozone, and so reduced the risk of contagion from the debt crisis, the agency said in an overall muted assessment of the long-term effects.

On Friday, the French-US rating agency Fitch said that it would issue a restricted default rating, and would then issue a new and probably higher rating of low investment grade for the new instruments.

Fitch also said that the rescue was an important step forward but warned that unless there was general economic recovery in the eurozone and progress on cutting budget deficits, further turmoil could not be ruled out and "downward pressure on sovereign ratings will persist."

Moody's explained that the European Union programme, together with support from big financial institutions in the Institute of International Finance IIF, "implies that the probability of a distressed exchange, and hence a default, on Greek government bonds is virtually 100 percent."

It said: "The magnitude of investor losses will be determined by the difference between the face value of the debt exchanged and the market value of the debt received. The IIF has indicated that investor losses are likely to be in excess of 20 percent."

The IIF said on Thursday that private sector investors "will contribute 54 billion euros from mid-2011 through mid-2004 and a total of 135 billion euros ($194 billion) to the financing of Greece from mid-2011 to end 2020."

But Moody's said that although the rescue package offered a number of benefits for Greece, "the impact on Greece's debt burden is limited."

The rescue raised the chances that Greece could stabilise and reduce its debt burden, and it helped the eurozone by "containing the severe near-term contagion risk that would have followed a disorderly payment default."

But "Greece will still face medium-term solvency challenges: its stock of debt will still be well in excess of 100 percent of gross domestic product for many years."

The agency also noted that Ireland and Portugal, which are also being rescued by the European Union and International Monetary Fund, would benefit from reduced loan rates.

But, "despite statements to the contrary, the support package sets a precedent for future restructurings."

The effect of the rescue strategy was therefore likely to have a neutral effect on perceptions of risk for people holding Irish and Portuguese debt, it said.

Moody's also said the positive effects of new powers for the EU EFSF financial stability fund on market sentiment had to be balanced against the negative effect of a precedent being set.

For eurozone countries which did not have the best credit ratings and had debt problems, on balance "the negatives will outweigh the positives and weigh on ratings in future," Moody's said.