Iraq's main Sunni Muslim political movement, the Iraqi Islamic party, says it is withdrawing from next month's general elections.
The party is widely supported in Sunni areas
Party leader Mohsen Abdul Hamid said the move was motivated by the authorities' refusal to delay elections so that all parts of Iraq could vote.
"We said we would take part if certain conditions were met, and they have not been," he told reporters in Baghdad.
The party had fielded a list of candidates earlier this month.
"Our party asked on 5 December that elections be delayed for six months using reasonable arguments," Mr Hamid said.
"The authorities concerned have refused to hear the voice of reason," he added.
Many Sunnis fear that violence in their areas means electors will be too intimidated to vote on 30 January.
Mr Hamid said six out of 18 provinces in Iraq would not be able to hold credible elections in the current circumstances.
With the pullout of the Iraqi Islamic party, correspondents say the only significant players left from the minority Sunni community are elder statesman
Adnan Pachachi and some other independent figures.