Popular Current leader Hamdeen Sabbahi said although President Mohamed Morsy loses his legitimacy, his legal legitimacy cannot be lost without a vote.
He denied calling for presidential elections earlier than scheduled.
Sabbahi told the television program Hona al-Qahera on Monday that he is disappointed about the way the country is being run, and that President Mohamed Morsy favors the interests of the Muslim Brotherhood rather than those of the country.
Sabbahi, along with co-founder of the Constitution Party Mohamed ElBaradei and Conference Party head Amr Moussa, leads the National Salvation Front, an alliance of caivil forces calling on Morsy to cancel the constitutional referendum and form a new constituent assembly to write a consensus constitution.
The front leaders had earlier said that Morsy started to lose his legitimacy after issuing a constitutional declaration on 22 November, which made his decisions unchallengeable.
“No one wants to exclude the Brotherhood. People reject wrong decisions, not the Brotherhood,” he said.