A British judge has described the “paranoid atmosphere” in Hong Kong as he explained his decision to step down from the territory’s top court.

Jonathan Sumption, along with another British judge, Lawrence Collins, last week resigned from Hong Kong’s court of final appeal (CFA).

Their decision came shortly after 14 people were found guilty of conspiracy to commit subversion in the biggest national security trial of pro-democracy activists.

Writing in the Financial Times to explain his decision to step down, Lord Sumption warned that Hong Kong was “slowly becoming a totalitarian state”.

The chief executive of Hong Kong, John Lee, has hit back at Sumption, suggesting the actions of the former British supreme court justice were politically motivated.

“His professional duty is to apply the … law in accordance with legal principles and evidence, whether he likes that law or not, not from his political stance,” Lee said.

A national security law passed in 2020 in response to mass protests in 2019 was “extremely illiberal”, Sumption told BBC Radio 4’s Today programme on Tuesday.

“The real problem, I think, in Hong Kong is the paranoid atmosphere there,” he continued. “This is said to be a response to the 2019 riots, but there were already laws perfectly capable of dealing with those. The object has become increasingly clear of the national security law was to crush peaceful political dissent, not just riots.”

Sumption said China was increasingly intervening in legal decisions in the territory.

“There is the problem that under the basic law, if China doesn’t like the court’s decisions, they can reverse them by what is called an interpretation, although it’s usually just a legislative intervention … It was initially unclear how frequently this would be used, but recent incidents have indicated that the Chinese are determined to use this provision in order to ensure that its opponents lose.”

At the end of May, 14 pro-democracy activists were found guilty of subversion in the largest application of the national security law to date. They included the former lawmakers Leung Kwok-hung and Helena Wong, the journalist turned campaigner Gwyneth Ho and others who joined the mass protests of 2019.

“I think that the picture is getting darker,” Sumption said. “The judgment on the 30 May against the 14 democracy activists was a major indication of the lengths to which some judges are prepared to go to ensure that Beijing’s campaign against those who have supported democracy succeeds.”

Lee, who assumed office as chief executive of Hong Kong in 2022, said in response to Sumption’s article in the FT: “This latest statement indicates that he doesn’t like the political situation in Hong Kong, but this is exactly the area he has told us in 2021 that should not be confused with the rule of law,” Lee said.

“His professional duty is to apply the … law in accordance with legal principles and evidence, whether he likes that law or not, not from his political stance.”

Overseas judges on Hong Kong’s top court have been a fixture of the city’s legal system, which unlike mainland China’s is derived from English common law, since the handover from British to Chinese rule in 1997.

The judges serve in a private capacity and are paid about £40,000 a visit, flying into Hong Kong to sit on certain cases. For decades, they were seen as lending prestige and expertise to a jurisdiction admired across Asia.