The International Swaps and Derivatives Association (ISDA) has ruled that the Greek bond swap constitutes a default.
This ruling will trigger the payment of credit default swaps, a form of insurance which buyers of bonds take out against the possibility of the bond defaulting.
Markets believe the payment of the CDSs will cost the banking system in the order of €3 billion. The ISDA ruling had been expected.
Earlier, Greece took a critical step to avoid bankruptcy with the unprecedented debt write-off deal, allowing the head of the euro zone's finance ministers to say that a second bail-out for the country was on track.
The event means that Greece is now set to repay debt soon due and has a second chance to rebuild its shattered economy, and that the euro zone has avoided default chaos that would have destabilised global markets.
EU officials welcomed the record debt swap with relief, but financial analysts warned that the Greek problem was simply contained, not buried.
"The Eurogroup considers that the necessary conditions are in place to launch the relevant national procedures required for the final approval of the euro area's contribution to the financing of the second Greek adjustment programme," Eurogroup head Jean-Claude Juncker said. The finance ministers' group held a conference call today, and are to meet on Monday in Brussels.
German finance minister Wolfgang Schauble said they had already released €35.5 billion in funds to cover euro zone participation in the bond swap, and that the remaining €94.5 billion, essentially loans to Greece, will probably be released next week. "We are not out of the woods but we have taken an important big step," he said.
EU Economics Affairs Commissioner Olli Rehn said the result of the swap offer was a "decisive contribution to financial stability in the euro area as a whole".
The head of the International Monetary Fund Christine Lagarde said in Washington: "This is an important step that will dramatically reduce Greece's medium-term financing needs and contribute to debt sustainability."
Almost 86% of private holders of Greek debt agreed to cancel half the money owed to them, enough for the swap to go forward. Greek officials said the swap would shave around €100 billion from its total debt of roughly €350 billion.
The successful debt deal was a key condition of the second bail-out, with Greece's parliament having already approved a raft of required measures to help balance the budget and liberalise the economy.
The swap announcement came shortly before official data showed that the Greek economy, in deep recession, has shrunk even more than expected. The state statistics agency said the economy had contracted by 7.5% in the fourth quarter of 2011, revising a previous 7% estimate. On an annual basis, this meant output shrank by 6.9%.
The high level of acceptance by private creditors met criteria for the EU and IMF to push on with an even bigger bailout for Greece, which said it would mop up remaining bonds affected by the exchange in the next two weeks. On the basis of full acceptance, Greece would secure a write-off worth €107 billion.
It is hoped that will keep Athens on track to reducing its debt to a sustainable level of about 120% of annual output by 2020.
Venizelos said all bonds issued under Greek law had been accounted for by virtue of legislation - known as collective action clauses - which enable Greece to force compliance on all investors based on the majority decision to participate. A government source later said the clauses had been activated.
Holders of bonds issued under foreign law, and of bonds issued by state companies guaranteed by Greece, will be given until March 23 to decide, the minister said, while warning that investors would be "naive" to expect a better offer.
Success for the debt swap is a vital step for Greece to avoid a default as early as March 20 when it has to repay more than €14 billion in debt.
The use by Greece of legal steps to force acceptance by recalcitrant bondholders could trigger anti-default insurance contracts, known as credit default swaps.
