"The decision of the Front, unanimously, is to boycott the elections," National Salvation Front member Sameh Ashour told reporters in Cairo Tuesday after a meeting of the alliance, which groups mainly liberals and leftists.
"There can be no elections without a law that guarantees the transparency of the electoral process... without a real independence of the judiciary," Ashour said as activists chanted slogans against Morsi's Muslim Brotherhood.
The decision came after NSF demands, including the formation of a new government "to save the country", had been ignored, Ashour added.
Leaders of the opposition alliance have been locked in heated debate over whether to take part in the staggered parliamentary elections, members said.
Already on Saturday, Mohamed ElBaradei, former head of the UN atomic agency, Nobel Peace laureate and a prominent member of the Front, had issued his own boycott call, arguing it was necessary "to expose sham democracy".
The NSF also shunned a national dialogue event set up by Morsi to establish guarantees for fair and transparent elections.