The UN facilitator for dialogue in Guinea, Said Djinnit said that after widespread consultations an "exceptional delay of four days for final adjustments before the elections" was decided upon.
Djinnit made the announcement at a press conference attended by leaders of the opposition Cellou Dalein Diallo and Sidya Toure, as well as the presidential majority.
Diallo, the west African nation's main opposition leader, has accused the president's camp and the electoral commission of conniving to rig the vote.
The delay is just one of several postponements of the poll, initially due to have been held within six months of the swearing-in of President Alpha Conde in December 2010.
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Disagreements between opposing factions on how the elections should be organised have sparked nationwide protests that have left more than 50 dead since 2011.
The last parliamentary elections in Guinea took place in June 2002 during the dictatorship of General Lansana Conte, who died in December 2008 after 24 years in power.
"On behalf of the international community, I am pleased to announce to all Guineans that all stakeholders in the electoral process have subscribed to this commitment.
"We are convinced that with this agreement, nothing will hinder the holding of free, transparent legislative elections on Saturday September 28, 2013, in Guinea," said Djinnit.