Greece will need far bigger debt relief than euro zone partners have been prepared to envisage so far due to the devastation of its economy and banks in the last two weeks, says a confidential study by the International Monetary Fund seen by reporters.

The updated debt sustainability analysis (DSA) was sent to eurozone governments late on Monday, hours after Athens and its 18 partners agreed in principle to open negotiations on a third bailout program of up to 86 billion euros in return for tougher austerity measures and structural reforms.

"The dramatic deterioration in debt sustainability points to the need for debt relief on a scale that would need to go well beyond what has been under consideration to date — and what has been proposed by the ESM," the IMF said, referring to the European Stability Mechanism bailout fund.