Bahrain's largest Shiite opposition group Wefaq said it would miss Wednesday's deadline to register for by-elections for the 18 parliamentary seats it quit in protest at a government crackdown on mostly Shiite demonstrators this year.

"The core issue is that the legislative authority does not exist any more," Khalil al-Marzouq, a former Wefaq lawmaker, explaining why the bloc would not contest the 24 September polls.

Troops from Saudi Arabia and other Gulf states went into Bahrain in March to help quell protests in the Sunni-ruled kingdom, in which at least 30 people, including four policemen, were killed, hundreds wounded and more than 1,000 detained.

King Hamad bin Isa al-Khalifa started a national dialogue in July to tackle the issues that led to the unrest in February and March, but Wefaq withdrew, saying it was under-represented and that the dialogue would not deliver the changes Bahrain needed.

The government of Bahrain, which hosts the U.S. Fifth Fleet, said reconvening a fully elected parliament was crucial to enact reforms proposed by the national dialogue and endorsed by the king, including enhancing the legislative body's powers.

The government said in a statement that it regretted Wefaq's decision to boycott the election.

"This is the opportunity for the people of Bahrain to vote in large numbers, thereby significantly boosting the reform process," Khalifa al-Dhahrani, speaker of the 40-seat parliament, said in the statement.

Although the king agreed to expanding the powers of the elected parliament last month, the broader powers of an appointed upper chamber are expected to be left intact.

Bahrain's Shiite majority demanded broader political liberties and an end to sectarian discrimination. A few Shiite groups, but not Wefaq, called for the monarchy to be abolished.

The government has said the unrest was sectarian and backed by non-Arab Shiite Iran. Bahraini Shiites denied this.

They say they are systematically denied access to land and jobs, and cite the naturalisation of foreign Sunnis, some of whom serve in the security forces, as proof of a policy of sectarian rule.

Bahrain has asked a panel of international law and human rights experts to investigate the protests and their aftermath.