Liberal and leftist parties have pulled out of a panel drafting Egypt's new constitution, they announced on Tuesday, accusing Islamists of monopolising the process to deliver its post-revolution charter.
The withdrawals from the panel will call its legitimacy into question, pushing the struggle between Islamists and secularists over the issue to crisis point.
"We announce our rejection of the way the constituent assembly was formed," Ahmed Said, the head of the liberal Free Egyptians party, told reporters.
Last week, the Islamist-dominated parliament voted for the panel to be made up of 50 lawmakers from the upper and lower houses of parliament, and 50 public figures.
But liberals argued that such a high proportion of legislators gave Islamists -- who control nearly three quarters of parliament-- too much control of the constitution.
They also say that having 50 members of the panel outside parliament gives a false guarantee of balance, as voters are free to chose Islamists outside the legislative body.
"We are talking about the constitution of Egypt, not one for a majority group," Said said during a news conference of key liberal and leftist parties.
With the current make-up, "the constitution will be drafted by political Islam... We refuse to betray the trust of the people," he said.
The popular uprising which toppled president Hosni Mubarak last year paved the way for the formation of dozens of political groups and movements of all political stripes, but it is the Islamists -- the most organised and with a wide network of supporters -- who have gained the most since the revolt.
The Muslim Brotherhood and the ultra-conservative Salafist Al-Nur party together went on to dominate both houses of parliament in recent parliamentary elections, sparking fears among secularists and the Coptic minority of restrictions on freedoms.
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Ziad Bahaa Eldin, an MP with the Egyptian Social Democratic Party, said members of his party had also withdrawn from the panel, joining several other high-profile liberals and leftists.
"This is not a partisan issue. This is about the future of the country and it is the right of all Egyptians to write their constitution... away from any partisan bias," he told reporters.
The liberal Wafd party, one of the oldest in Egypt, has yet to decide whether it will pull out.
Shortly after the news conference, the heads of Egypt's main parties met military ruler Field Marshal Hussein Tantawi to discuss the crisis.
Meanwhile, Cairo's Administrative Court on Tuesday began looking into the legitimacy of the constitutional panel after lawsuits were filed by several legal experts who argue that a constitution cannot be drafted by those whose role it will define.
Dozens of protesters outside the court chanted slogans against Islamist domination of the panel, saying: "The constitution is for all Egyptians."
Protesters, including liberal and leftist activists and lawyers, are calling for the constituent assembly to be made up entirely of non-parliamentarians.
The constitutional struggle has piled more pressure on the powerful Muslim Brotherhood, amid a simmering crisis between the Islamist group and the ruling Supreme Council of the Armed Forces (SCAF) for backing the current government.
The Muslim Brotherhood's political arm, the Freedom and Justice Party, has been pressuring the military to sack the cabinet -- which it accuses of stalling the revolution -- and to appoint an FJP-led government.
But the SCAF has stood by the cabinet and its head Kamal Ganzuri, lashing out at the Islamists for their demand.
On Tuesday, hundreds of Muslim Brotherhood youth members protested in Cairo to demand Ganzuri's dismissal.