The Council of State, Greece's highest administrative court, is due on Friday to rule on whether Sunday's referendum is in violation of the Constitution.
The court was due to convene around to noon to assess a petition submitted on Wednesday by a lawyer and a civil engineer challenging the plebiscite's legality.
The petitioners claim that the plebiscite violates the Constitution by posing a question regarding "public finances" and further uses complicated terms that may not be understood by many voters.
On Wednesday, the head of the Council of Europe, Europe’s top human rights institution, told The Associated Press that Greece’s referendum would fall short of international standards if held as planned on Sunday.
The vote “has been called on such a short notice, that this in itself is a major problem,” Council of Europe Secretary General Thorbjorn Jagland told the AP on Wednesday. “And also the fact that the questions that are put to the people ... are not very clear.”