Egypt's main opposition group, the National Salvation Front (NSF), will boycott the forthcoming parliamentary elections, a spokesman says.
Spokesman Sameh Ashour said the decision was taken because of concerns the polls would not be free and fair, AFP news agency reported.
It comes days after President Mohammed Morsi announced the timing of the elections, to be held over four dates.
Judges dissolved the previous assembly, saying polls were unconstitutional.
The first round of voting in Cairo and four other provinces is due to be held on 22 April.
In the last elections, in January 2012, Islamist parties won an overwhelming majority, with the Freedom and Justice Party of Mr Morsi's Muslim Brotherhood movement taking the biggest share.
The lower house was dissolved in June after the Supreme Constitutional Court ruled that one of the laws under which the elections were fought was not legitimate.
Egypt is sharply divided between Islamists and their liberal and secular opponents and a boycott of the polls threatens to deepen the split, say correspondents.
Mr Morsi announced new polls last weekend. NSF leader Mohamed ElBaradei swiftly called for a boycott, branding fresh elections an "act of deception".
Mr Ashour said the NSF would not contest the polls under an election law which critics said favours Islamists.
"There can be no elections without a law that guarantees the fairness of the election process and a government that can implement such a law and be trusted by the people," he said in a televised news conference.
He said the NSF had unanimously decided to endorse Mr ElBaradei's call.
Mr Ashour said the alliance would also stay away from a meeting to promote national dialogue called by President Morsi, describing it as an insult to protesters killed in recent clashes.
More than 70 people were killed in violence between security forces and protesters following the second anniversary of the revolution which forced Hosni Mubarak from power.