US secretary of state Mike Pompeo has notified Congress that the Trump administration no longer regards Hong Kong as autonomous from mainland China.
It follows Beijing's plan to impose a controversial new security law on the territory.
The security law was "only the latest in a series of actions that fundamentally undermine Hong Kong's autonomy and freedoms," Mr Pompeo said.
"It is now clear that China is modelling Hong Kong after itself," he added.
The move does not carry any immediate penalties, which would have to be decided by President Donald Trump in consultation with Congress.
But the administration sees it as putting China on notice that Hong Kong’s perks are in jeopardy.
Mr Pompeo said: “Hong Kong does not continue to warrant treatment under United States laws in the same manner as US laws were applied to Hong Kong before July 1997.”
The notice brings the future of Hong Kong squarely into the administration’s numerous battles with China that have put the world’s two largest economies at odds.
Relations have worsened over US allegations that Chinese authorities sought to cover up the coronavirus pandemic and pressured the World Health Organization from taking early action to combat it.
That has added to long-standing tensions over trade, human rights, religious freedom and the status of Taiwan.
Mr Pompeo’s certification comes amid calls in Congress and elsewhere for the US and others to react against Beijing’s move to impose Chinese national security laws over the territory.
“Beijing’s disastrous decision is only the latest in a series of actions that fundamentally undermine Hong Kong’s autonomy and freedoms and China’s own promises to the Hong Kong people,” Mr Pompeo said.
He continued: “No reasonable person can assert today that Hong Kong maintains a high degree of autonomy from China, given facts on the ground.”
After sending the notification to Capitol Hill, Mr Pompeo spoke by phone with UK Foreign Secretary Dominic Raab. The two men agreed China “must honour its commitments and obligations under the Sino-British Joint Declaration”.
“Both agreed the international community must support the people of Hong Kong and respond to Beijing’s continued erosions of Hong Kong’s autonomy,” the US State Department said in a statement about the call.