Nasserist Hamdeen Sabbahi and reform campaigner Mohamed ElBaradei called for boycotting the current constituent assembly on the grounds that it is incompetent during a meeting held on Friday.
A statement by the two prominent politicians and others is expected to be released shortly, to voice their objection to the preliminary results of the work of the constitution drafting body.
Political forces which have called for the boycott accuse the assembly of throwing away the economic and social rights of Egyptians, asked members of the constituent assembly affiliated with national parties or forces to withdraw.
"President Mohamed Morsi is responsible for the shortcomings of the constituent assembly, for failing to meet his promise of forming a constituent assembly representative of all members of the Egyptian society," said Journalist and Co-founder of the pro-democracy Keyafa movement Abdel-Halim Qandil who was present at the meeting.
Egypt's beleaguered Constituent Assembly has already suffered a number of withdrawals since mid-June, when the 'Egyptian Bloc' parties – including the Free Egyptians, the Egyptian Social Democratic Party and the leftist Tagammu Party – initiated a mass walk-out, which was followed by the Karama Party, the Socialist Popular Alliance Party and the Democratic Front Party.
Their stated reason for resigning from the assembly was to allow greater representation for women, young people and Coptic Christians, while also registering their objection to perceived "Islamist monopolisation" of the constitution-writing committee.
The troubled assembly still faces the risk of dissolution by court order on grounds that it was drawn up by the People's Assembly, the since-dissolved lower house of Egypt's parliament.
In October, Egypt's Supreme Administrative Court is set to rule on the assembly's constitutionality, or lack thereof.