Hong Kong's leader has defended China's plans to grant her the authority to appoint judges in cases brought under a new national security law, amid growing concerns that the provision would erode the semi-autonomous city's vaunted judicial independence.

Carrie Lam's comments on Tuesday came as the European Union warned China it would face "very negative consequences" if it presses ahead with the legislation banning what Beijing termed secession, subversion, terrorism and collusion with foreign forces in Hong Kong.

The law is being drafted by the Standing Committee of China's National People's Congress, the country's top legislative body, and local media say the legislation could be passed by the end of June.