The Fitch international ratings agency has cut its long-term rating on Greek debt bonds by two notches -- from "triple C" to "C."
Fitch says it now considers a Greek debt default to be "highly likely in the near term."
The move follows the announcement on February 20 of a Greek debt swap deal with private creditors -- a plan in which private banks and other financial institutions are being asked to write off some 107 billion euros of debts they are owed by Athens.
In June, Fitch said it would consider Greece to be in a "restricted" default if the deal went ahead.
Greek Finance Minister Evangelos Venizelos has said the debt write off -- along with a 130 billion euro in bailout loan agreed by eurozone countries -- has helped Greece avoid a "nightmare scenario."