In the statement, issued in the early hours of Sunday Hong Kong time, they say the integrity of "One Country, Two Systems" is hanging by a thread, and warn that draconian laws will escalate protests here.

The signatories are led by Hong Kong's last colonial governor, Chris Patten, and the former British foreign secretary, Malcolm Rifkind. Other signatories include five US senators and lawmakers from Europe, India, Indonesia, Malaysia and Myanmar, as well as New Zealand, Canada and Australia.

In the statement, they describe the proposed national security law as a "comprehensive assault on the city's autonomy, rule of law, and fundamental freedoms. The integrity of one-country, two-systems hangs by a thread.

They add: "It is the genuine grievances of ordinary Hongkongers that are driving protests. Draconian laws will only escalate the situation further, jeopardising Hong Kong’s future as an open Chinese international city.

"If the international community cannot trust Beijing to keep its word when it comes to Hong Kong, people will be reluctant to take its word on other matters. Sympathetic governments must unite to say that this flagrant breach of the Sino-British Joint Declaration cannot be tolerated."

Rifkind said: “This is the most serious threat to the people of Hong Kong that there has been from the Chinese Government since 1997. The people of Hong Kong need, and deserve, our support."

Beijing has long dismissed comments by foreign politicians and diplomats on Hong Kong as unwarranted interference in China's domestic affairs.

The central government announced this week that the National People's Congress would vote on a draft of the proposed security law on Thursday.

Hong Kong and central government officials say the law is needed to combat what they call separatism and terrorism in the SAR.They say it will not undermine rights and freedoms in Hong Kong.