Hossam Eissa officially resigned on Thursday from the Dostour Party which oppositionist Mohamed ElBaradei founded.
Eissa who was a leading member of the party said he quit because he could not deal with liars. He did not mention any names.
"My decision is final," Eissa, a constitutional law professor, said.
"As of today, I no longer consider myself a member of the Dostour Party. Nothing whatsoever links me to the party anymore."
Eissa said he tendered his resignation a few days ago but ElBaradei asked him to reconsider his decision and return to the party to finish structuring it.
It became apparent yesterday that my reasons for resigning were still valid, he added.
Eissa said ElBaradei would announce that young party members will take over the party and its duties as of the coming September.
He also urged the party's youths to stand their ground and support ElBaradei, head of the party.