A British judge who recently resigned from Hong Kong's top court has warned the territory is "slowly becoming a totalitarian state" with its rule of law "profoundly compromised."

Jonathan Sumption is one of two British judges who resigned shortly after a landmark verdict in which 14 prominent democratic activists were convicted for subversion amid a national security crackdown on dissent.

Some lawyers say the resignations challenge a long-held assumption that having foreign jurists on the court of final appeal helps protect the city's international image after China imposed a national security law on Hong Kong in 2020 in response to mass pro-democracy protests.

Explaining his eventual decision to resign from Hong Kong's court of final appeal, Mr Sumption said Hong Kong authorities were "paranoid" about political dissent.

"Hong Kong, once a vibrant and politically diverse community is slowly becoming a totalitarian state," Mr Sumption wrote in a rare and strongly worded opinion piece for the Financial Times on Monday.

"The rule of law is profoundly compromised in any area about which the government feels strongly."

The national security law, which criminalises secession, subversion or terrorism with up to life in prison, has led to hundreds of arrests.

The largest national security case involved 47 pro-democracy legislators and political activities who were trialled for "subversion" and their involvement in organising an unofficial legislative council primary election to choose pro-democracy candidates in 2020.

Fourteen of those pro-democracy Hongkongers were found guilty.

Mr Sumption said the national security law "severely limits" judges' freedom of action, highlighting Beijing's power to intervene and interpret laws if "China does not like the courts' decisions."

The Hong Kong government said it "strongly disapproves" of Mr Sumption's remarks and called it a "betrayal against Hong Kong's judges."

Beverley McLachlin, a Canadian judge who announced her resignation on Monday, said she was leaving the top court to spend more time with her family, but has "confidence in the members of the court, their independence, and their determination to uphold the rule of law."

Hong Kong’s leader John Lee thanked Ms McLachlin for her “objective assessment of the city’s rule of law.”

Four Australian judges – Patrick Keane, Robert French, William Gummow and James Allsop – continue to serve as non-permanent judges on Hong Kong’s top court despite concerns over their judicial independence.

Former Hong Kong parliamentarian Ted Hui has urged them to resign "to maintain the integrity and value of the Australian legal profession."

He said the recent Article 23 security law further erodes legal protections, allowing for detention of up to 16 days without access to a lawyer.

"The rule of law is not only compromised, it's decimated," Mr Hui told the ABC.

Chung Ching Kwong from the Inter-Parliamentary Alliance on China said the presence of foreign judges on Hong Kong's top court gives it a false sense of legitimacy.

"It is time to leave and admit that there is no moderate influence possible in the legal system in Hong Kong," she said.

"I know the judges are paid top dollars, but I believe their role is worth more than that money."

Only one Australian judge, James Spigelman, stepped down in 2020 citing concerns over the national security law.

The ABC has approached the Australian judges for comment.

ABC/Reuters