Two more defendants accused of conspiring to commit subversion over unofficial Legco primary polls in 2020 have admitted to the charges, as the trial started on Monday.

Former district councillor Ng Kin-wai and businessman Lam King-nam pleaded guilty to their charges.

The 47 defendants are accused of attempting to win enough seats in the legislature to paralyse the government.

Sixteen defendants have earlier pleaded not guilty, among them former lawmakers Helena Wong, Lam Cheuk-ting and Leung Kwok-hung, and former district councillors Kalvin Ho and Clarisse Yeung.

Lawyers for the defendants told the court they need more time to prepare for the case, saying they'd recently received a large amount of new information.

The prosecution, meanwhile, said four of the 31 defendants who pleaded guilty will be summoned to give evidence.

The national security trial is expected to last 90 days. It's heard by High Court judges Andrew Chan, Alex Lee and Johnny Chan.

The hearing is taking place at the West Kowloon Magistrates' Court, where bigger courtrooms are available.