A Greek referendum on the bailout deal would be a vote on its euro zone membership, a Finnish minister said in a TV interview on Tuesday.
Greece's Prime Minister George Papandreou on Monday called the unexpected referendum, citing the need for wider political backing for the fiscal measures and structural reforms demanded by international lenders.
The vote is seen bringing uncertainty back to markets and threatening Europe with a new crisis after euro zone leaders only last week agreed on the last-minute bailout deal.
"The situation is so tight that basically it would be a vote over their euro membership," Alexander Stubb, the Finnish minister of European affairs and foreign trade, said in an interview with Finnish broadcaster MTV3.
Stubb said Greece had committed to the summit decision.
"Greece has committed to a new program which includes structural reforms. All of a sudden, if they vote against those reforms, then Greece is the one who violated the agreement," he said.
He added no bailout aid would be deployed if Greece dodged from the required reforms.
The latest opinion poll showed a majority of Greeks took a negative view of the bailout deal.
Euro zone leaders last week agreed to hand Greece a second, 130-billion-euro bailout with 50-percent write-down on its debt.