A Bahraini court has banned Al-Wefaq, the country's main Shi'ite opposition group, for three months just before before a parliamentary election is due to be held.
Manama administrative court ruled that the party violated the law on associations, meaning that it suspended all the party's activities for three months.
Earlier, Al-Wefaq, which was founded in 2002, announced it was boycotting national and local elections scheduled for 22 November. "Elections that don't lead to a power transition within a constitutional monarchy will instead entrench dictatorship," it said.
The EU member states' ambassadors in Bahrain expressed "deep disappointment" at the decision.
After the crackdown on the pro-democracy uprising in 2011, led by Saudi forces, Bahrain has plunged deeper into sectarian conflict between the wealthy ruling Sunni-al-Khalifa minority and the Shia majority.
King Sheikh Hamad bin Isa Al-Khalifa has pledged to implement recommendations by an independent commission of inquiry but reforms are progressing slowly and reconciliation talks have stalled. Violence between riot police and protesters is a weekly occurrence. Decrees approved by King Hamad include up to seven years in jail for criticising him. All protests, sit-ins and gatherings in Manama are banned indefinitely.
Bahraini authorities have also stepped up detention of pro-democracy activists such as Nabeel Rajab, who was arrested over Twitter remarks alleging that the kingdom's security institutions were the first ideological incubator for the Islamic State (Isis).