European Union officials say Greece would lose access to more than 16 billion euros ($18 billion) in financial support if its bailout program expires at midnight (2200 GMT).
The officials, who spoke on condition of anonymity because contacts about the program were still ongoing, said three sources of money would disappear in the event of no agreement to extend the bailout.
These include 1.8 billion euros from the EU's financial stability fund, 10.9 billion euros from a Greek bank rescue fund, and a further 3.4 billion euros in central bank profits.
Greece can apply for some other form of assistance, but this could take weeks to organize. In that case, an assessment would first have to be made on whether Greece is eligible, what kind of terms the new package would function under and the kinds of reforms that Athens would undertake in return.