Bahrain's biggest Shia opposition bloc says it is planning to pull out of talks set up by the Sunni-led government in the wake of pro-democracy protests.
A spokesman for the group, al-Wifaq, said the national dialogue was not serious and they would not be attending Sunday's session.
The bloc had complained that the opposition had been granted too few seats in the talks.
The dialogue was launched on 2 July.
An al-Wifaq spokesman said the decision to abandon the national dialogue would be put to the Wifaq leadership for ratification.
"We have tried but without success to make it a serious dialogue," the spokesman told the French news agency, AFP.
Announcing the start of the talks earlier this month, King Hamad Bin Issa Al Khalifa said all options were on the table.
The discussions follow months of anti-government protests in the Gulf kingdom that left more than 30 people dead.
Hundreds of opposition supporters have been jailed.
Al-Wifaq has said it is not seeking the fall of the government in Shia-majority Bahrain but is seeking reform.