Germany's central bank, the Bundesbank, expects Greece to need a new bailout plan in early 2014.
A new report in Der Spiegel magazine says it obtained a Bundesbank document expressing doubts that Greece will implement all necessary reforms.
The document cites "political restraints" for the reason Greece will not meet lenders' terms.
It predicts that Greece will need a new aid program in 2014 to deal with its ballooning debt crisis. The document also expressed worries about the risk of the current bailout deal for the Greek government, which it says has performed unsatisfactorily.
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German Chancellor Angela Merkel has downplayed the prospect of further help for Greece ahead of elections in late September, which she is expected to win.
Merkel has said that the bailout program for Greece is on track despite the recent report suggesting otherwise.
Opposition parties have attacked the chancellor after the report was released, saying she lied about the success of the bailout.
Greece has already received nearly 240 billion euros from Europe and is expecting the last 10 percent of that at the end of 2014.
Despite the bailout funds, the Greek unemployment rate continues to rise, and is now at a record 27.6 percent.