Germany fears 'full-blown bankruptcy' in eurozone

In the starkest language yet by a European official, the German minister called for additional aid to be made available to Greece, adding that private banks should participate in the cost of the Greek rescue.

EU officials and member states are understood to be currently working on a second bail-out agreement for Greece, in addition to the €110 billion pledged last year, with estimates suggesting the new aid package could total €60 billion.

Finance ministers are expected to reach an agreement on 20 June, just three days before a summit of European leaders, with Schaeuble suggesting that private creditors should be made to wait an extra seven years before repayment of their existing Greek loans.

"Any agreement on 20 June has to include a clear mandate - given to Greece possibly together with the IMF - to initiate the process of involving holders of Greek bonds. This process has to lead to a quantified and substantial contribution of bondholders to the support effort, beyond a pure Vienna initiative approach," reads the letter.

"Such a result can best be reached through a bond swap leading to a prolongation of the outstanding Greek sovereign bonds by seven years, at the same time giving Greece the necessary time to fully implement the necessary reforms and regain market confidence."

The ECB is strongly opposed to a restructuring of Greek debt however, partially because the bank has bought large quantities of Greek bonds over the past year in order to stabilise markets.

ECB executive board member Lorenzo Bini Smaghi on Monday said Greece had marketable assets worth €300 billion, roughly equal to the country's debts, and was therefore not bankrupt.

"Greece should be considered solvent and should be asked to service its debts," he told journalists. "Restructuring should only be the last resort ... when it is clear that the debtor country cannot repay its debts."

The battle between Germany and the ECB is likely to play out in the coming days, with the current leadership uncertainty at the IMF also a complicating factor however.

In his letter, Schaeuble called on the international lender to maintain its support for Greece.

At the same time, unease is growing within Angela Merkel's Christian Democratic Union over further aid to Greece.

The German Chancellor is on Wednesday set to defend her plans in front of increasingly mutinous MPs, who feel they are being bounced into backing a further Greek bail-out.

The confrontation comes a day after Merkel met US president Barack Obama in the White House, with the American leader warning that the European debt crisis cannot be allowed to threaten the global economy.