Egypt's presidency has announced that the referendum on the a draft constitution will be held over two days.
The announcement, made on Wednesday, stated that the vote will be held on December 15 and 22.
It came just hours after the National Salvation Front (NSF) alliance of opposition parties called for citizens to vote "no" on referendum, and has set conditions that, if unmet, would result in a boycott of the poll.
The NSF demanded a full judicial supervision of the process, and that international and local NGOs be allowed to monitor the poll. It also called for voting to take place on a single day.
Al Jazeera's Mike Hanna, reporting from Cairo, said that the decision to spread the vote out over two days could bring the opposition "back to square one", although the NTS confirmed late on Wednesday that it would not boycott the vote.
However, despite the opposition's uncertain path, Hanna said that a constitutional victory for Morsi and the Muslim Brotherhood is far from certain.
"One must look at the figures. If you see that President Morsi, who is a supporter of that constitution - as are his followers - he became president with some 51 per cent of the vote. So clearly, there's not an overwhelming support for President Morsi himself, and by extenuation, the Freedom and Justice Party, and, indeed, the Muslim Brotherhood, of which the Freedom and Justice Party is the political arm," said Hanna.
"So there can be no speculation as to which way this particular vote is going to go."
Meanwhile, the key opposition politicians said on Wednesday that they were prepared to take part in national unity talks with the army.
Amr Moussa, Mohamed ElBaradei, leftist Hamdeen Sabahy and a Wafd party leader Mounir Fakhry Abdel-Nour said they would attend unity talks hosted by the army, but the army said on Wednesday it had indefinitely postponed the dialogue due to a low level of response for attendance.
The draft constitution, approved by the constituent assembly last month, has become the focus of Egypt's worst political crisis since President Mohamed Morsi's election in June.
Rival mass rallies held by both supporters and opponents of President Morsi have become almost a daily occurrence in Cairo, and clashes between the two groups killed at least seven people and injured hundreds more last week.
The crisis has necessitated a ramping up of security around the presidential palace, which has been the focal point of anti-Morsi protests.
Egyptians abroad, meanwhile, have already begun voting in the referendum on the new constitution, state media reported on Wednesday.
Voting was taking place at Egyptian embassies abroad, with more than 500,000 Egyptians expected to cast their votes in 150 countries.
Wednesday's developments come after rival rallies were held again on Tuesday, with anti-Morsi protesters outside the presidential palace calling for a boycott of the referendum. Pro-government supporters also held a demonstration, expressing their support for Morsi's decision to hold the referendum.