A spokesman for the opposition coalition, Aboubacar Sylla, said the election commission chief had announced the date of May 12 without the necessary two-thirds majority approval of its members.
“This date is null and void,” Sylla told reporters.
The rejection risks bringing further delays to unblocking more than $200 million in aid to the West African state, whose transition to full democracy has been fraught with political bickering.
An election commission member told Reuters only about half of the 25-member body had agreed on the date proposed by its head, Bakary Fofana.
Sylla also said opposition parties no longer had any confidence in Sabari and Waymark, the two firms that have been chosen by the government to update the voter list.
Guinea is currently run by President Alpha Conde, who was elected in 2010 in the first free vote after 50 years of one-man rule. The army seized power in 2008 after the death of veteran leader Lansana Conte and Guinea endured two years of often violent military rule.
The parliamentary poll was due in 2011 but has already been delayed four times.
The European Union, one of Guinea’s major donors, warned in November that it needed a credible and detailed timeline for the election to unblock about 174 million euros ($226.90 million).
The political deadlock has also contributed to doubts amongst investors, who are keen to tap into the country’s vast iron ore reserves.
In a sign of growing tension, at least three people were killed and 106 wounded in clashes on Monday night in southern Guinea between opposition militants and government supporters.