The scheme, intended to put Greece's mountainous debt on sustainable footing and unlock cash to avoid bankruptcy, had a maximum budget of 10 billion euros to buy back the bonds, which trade at a huge discount to face value.

But Athens said it would need 11.29 billion euros in funding to be able to buy back all the bonds investors offered. It plans to pay an average price of about 33.8 percent of face value on all series of bonds - a price that is well above estimates given at the time the scheme was agreed last month.

If international lenders were to approve the extra funding, the settlement date for the transaction will be December 18.

Completion of the debt buyback scheme, which would account for half of a broader debt relief package that lenders agreed for Athens last month, will ensure the IMF stays on board with the country's international rescue.

The Greek debt agency did not provide details on the buyback's impact on debt sustainability.

(Reporting by George Georgiopoulos and Harry Papachristou; Editing by Catherine Evans)

• A Permanent Economic Emergency