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.
For Hong Kong's democracy movement, China's intervention is a challenge to freedom of expression and judicial independence, but for Beijing the bigger picture is paramount.
All talk of independence is seen as threatening and elsewhere in China, separatism is a crime and campaigning for independence results in a lengthy jail term.
To allow elected members of Hong Kong's legislature to use such a high-profile public platform to insult China and talk of a Hong Kong nation was unthinkable.
Hong Kong's courts do still uphold the freedoms promised when Britain handed the territory back to China nearly two decades ago.
But this intervention from Beijing is a reminder that China is determined to decide the limits of those freedoms.