The Sino-British Joint Declaration was signed in 1984 to settle the future of Hong Kong. Under this declaration, Britain agreed to return Hong Kong to China on July 1, 1997. It also stated that China's basic policies regarding Hong Kong "will remain unchanged for 50 years," including the promise that the city would retain a high degree of autonomy.

The basic policies are detailed in the annex of the declaration and are part of the Basic Law of Hong Kong, which was approved by the National People’s Congress in April of 1990 and took effect on July 1, 1997. The remaining parts of the Sino-British Joint Declaration outline the process of the handover of Hong Kong from Britain to China to ensure a smooth, proper and effective implementation of the policies. The declaration, a copy of which is with the United Nations, is legally binding today.

However, in the light of recent unrest in Hong Kong, China's Foreign Ministry has stressed that the declaration does not provide for Britain's rights and responsibilities after the handover. This means Britain has no right to interfere in the city's affairs after July 1, 1997. China's basic Hong Kong policies, elaborated in Annex I, are a “unilateral” declaration on the part of China. Britain had no say in these basic policies.

China has made it clear since the 1980s that how it handles Hong Kong after the handover is a domestic matter, and that it was only “out of cordiality” that it agreed to make "some introductions" about its policies for the city.

Liao Fan, a legal expert with the Chinese Academy of Social Sciences, emphasized in an interview with CGTN that the most important and fundamental principle of the international law is the "mutual respect of sovereignty and territorial integrity." He said Britain's interference in Hong Kong's internal affairs with the Chinese mainland is against the fundamental principle.

China has repeatedly stressed its sole sovereignty over Hong Kong and reiterated it will not let any other country interfere in its domestic affairs.