Officials on state-run television said the vote, originally scheduled for only this weekend, now will be Saturday and Dec. 22, with half of the country's governorates voting each day, Egypt Independent reported.
The decision was made after most judges refused to supervise the vote in protest of President Mohamed Morsi's constitutional declaration increasing his own power and undermining the judiciary, including an edict that his actions were not subject to judicial review, which eventually was rescinded following massive protests.
Opponents charge the draft constitution was drafted by Morsi's Islamist allies to the exclusion of others' views, and tramples on human and civil rights.
Judges Club leader Ahmed al-Zend said during a news conference Tuesday a survey indicated 90 percent of judges and prosecutors across Egypt wouldn't participate in the constitutional referendum.
Prosecutor General Talaat Abdallah issued a memorandum on Tuesday appealing to prosecutors nationwide to supervise the referendum, the Independent said.
Egyptians living abroad began voting Wednesday on the draft constitution, the Independent said. The 586,000 eligible voters have four days to cast ballots at designated embassies and consulates.
Meanwhile, Egypt's army chief called for unity talks Wednesday as clashes related to the constitutional referendum persisted.
The talks, which would involve Morsi, judges and "political parties and forces," would seek to resolve the current political crisis, Col. Gen. Abdel-Fatah el-Sissi said in a statement released by his spokesman.
Sissi is Defense minister, the Egyptian armed forces commander in chief and chairman of the Supreme Council of the Armed Forces junta that ruled Egypt after President Hosni Mubarak was ousted Feb. 11, 2011. The junta gave up power June 30 at the start of Morsi's presidency.
Morsi spokesman Yasser Ali told Egypt's CBC satellite channel Morsi would attend the meeting, which Sissi's spokesman said would not be a political dialogue -- only Morsi could order that -- but an opportunity to offer a "message of comfort" to Egyptians.
Also invited were Morsi Cabinet members, "revolutionary youth," Islamic and Christian representatives, media leaders, and "artists, laborers and farmers," the statement from Sissi's spokesman said.
The talks would take place at the military's Air Defense Hall at a sports complex in New Cairo outside Cairo at 4:30 p.m. local time (9:30 a.m. EST), the statement said.
The Muslim Brotherhood, the Islamist organization Morsi once led, said it would participate in the meeting. Some opposition members, who contend the draft charter restricts freedoms and gives Islamists vast political influence, said they would not participate in a non-political gathering.
The main secular opposition coalition, the National Salvation Front, led by Nobel Peace laureate and former International Atomic Energy Agency chief Mohamed ElBaradei, said it would announce Wednesday if it would call for a "no" vote on Saturday's referendum or an outright boycott.
An Egyptian court said late Tuesday it had no authority to rule on the referendum. The administrative court of Egypt's Council of State said Morsi's pronouncement to move forward with the referendum was a presidential "sovereign decision" and could therefore not be overruled by the court.
The judges' decision to boycott the referendum may influence opponents deciding whether to campaign for votes against the disputed charter or to boycott the referendum entirely, The New York Times said.
Meanwhile, Morsi's government Tuesday postponed a $4.8 billion International Monetary Fund loan intended to help Egypt avert financial collapse.
The government sought the delay in the loan deal, which it struck last month, "in light of the unfolding developments on the ground," the IMF said Tuesday.