Egypt's presidential office has not appointed Mohamed ElBaradei as interim prime minister despite an earlier announcement that he would be sworn in on Saturday night.
The former head of the International Atomic Energy Agency had been widely expected to take up his post three days after the ousting of president Mohamed Morsi.
Speculation had been rife for several days that ElBaradei would head the transitional government alongside the acting president, Adly Mansour.
But the presidential office backed away from an earlier announcement that the pro-reform leader would be installed.
Ahmed el-Musilamani, a spokesman for Mansour, told the media that consultations were continuing, denying that the appointment of the Nobel Peace laureate was ever certain.
However, reporters gathered at the presidential palace were ushered into a room where they were told by officials to wait for the president who would arrive shortly to announce ElBaradei's appointment.
A senior opposition official, Munir Fakhry Abdelnur, said that the reversal was because the ultra-conservative Salafi al-Nour party objected to the appointment and mediation was underway.
ElBaradei, who leads the National Salvation Front, an alliance of liberal and leftwing parties, had been expected to be formally sworn in at the presidential palace.
The politican, who has lived in the west for decades, had been widely tipped to take up the position following the ousting of Morsi.
He has played a key role in the background of the current turmoil, meeting General Abdel Fatah al-Sisi on Wednesday.
"ElBaradei is our first choice," a source close to the army said earlier this week.
"He's an international figure, popular with young people and believes in a democracy that would include all political forces. He is also popular among some Islamist groups."
ElBaradei, who won the Nobel peace prize for his work with the nuclear agency, has used his Twitter feed to renounce violence "of all forms".
However his appointment is unlikely to appease Islamists who have regarded him as too liberal and have resolved to stay on the streets until Morsi is returned to power.
ElBaradei, 71, has also defended the army's takeover and supported the temporary arrest of senior members of the Muslim Brotherhood.
In an interview with the BBC, he said: "It is not the army who took over, it is the army who acted on behalf of the people."
ElBaradei has said the Muslim Brotherhood should be part of the political process but has supported the arrest of senior figures belonging to the movement, saying they had been "plotting".