Beijing has made an unprecedented intervention in Hong Kong politics to block two lawmakers from taking office.
Pro-independence elected lawmakers Sixtus Leung and Yau Wai-ching have refused to pledge allegiance to Beijing when being sworn in.
Beijing has now interpreted a section of Hong Kong law to mean any official who does not swear the oath properly cannot take office, said state media.
The move comes after weeks of chaos in the Hong Kong legislature.
There were also protests, and some scuffles, in Hong Kong on Sunday night, with at least four arrests.
Hong Kong's Chief Executive CY Leung said his government would "fully implement" the ruling.
Hong Kong is semi-autonomous under the "one country, two systems" framework in place since it was returned to China in 1997.
But its mini constitution, the Basic Law, states Beijing still has the final say in how to interpret its laws.