Lawyer Michael Fisher wrote one of the main university textbooks on Hong Kong’s legal system in 2019, but it is already out of date.

Last year, China imposed a national security law on the territory, under which one of his former students from his years teaching at Hong Kong universities has already been arrested.

“I only released it [the book] two years ago, now it needs to be completely rewritten,” said Fisher.

The introduction of the security law presaged sweeping changes to Hong Kong’s political and civil society aimed at suppressing dissent and more closely integrating the city with the mainland after anti-government protests in 2019.

Many businesspeople in Hong Kong still believed the changes would spare the territory’s courts, which are modelled on the UK’s common law legal system and are seen as integral to the city’s role as an international financial centre.

But one year on, analysts said the law was rapidly undermining legal norms in the city, such as the presumption of innocence and the right to bail in cases involving the security law. Judges were also being shown to have less sway over these cases.

Rushed through Beijing’s rubber-stamp parliament and imposed on Hong Kong, the law ruptured the firewall that had separated the territory’s legal system from that of mainland China, allowing suspects to be tried across the border for the first time.

Its vaguely defined crimes, including subversion and colluding with foreign forces, are punishable with up to life imprisonment. Critics have said it threatens the freedoms guaranteed to Hong Kong on its handover from the UK to China in 1997. Those charged under the law fear they may not get a fair trial.

“The lowest rate for a barrister for one day is HK$75,000 (US$9,655),” the spouse of a politician who was denied bail under the law told the Financial Times, explaining why the couple had yet to decide on a lawyer despite an upcoming court date. “[But] it’s not going to make a difference anyway, because the case is entirely political.”

A total of 117 people have been arrested under the law, including a 15-year-old, while 64 have been charged, according to Hong Kong’s Security Bureau. Many were denied bail.

In another break with the city’s common law tradition, a judge recently ruled in a case involving the security law that there was no constitutional right to a jury trial.

No security law case has yet concluded, but legal experts said preliminary rulings showed the judiciary only had so much power to mediate its impact.

“The national security law makes clear that it has the overriding status,” said Bing Ling, a Chinese law expert at the University of Sydney.

Once unheard of, Chinese state media and officials are also increasingly commenting on Hong Kong legal cases.

When a judge briefly let pro-democracy media mogul Jimmy Lai out on bail in December, China’s state media threatened that if Hong Kong’s judiciary could not “properly” handle the case, the mainland courts would take over. After an appeal, Lai’s bail was revoked. He has since been jailed in Hong Kong on separate charges.

Analysts said over-reach by the police also weighed on the legal system. A 37-year-old man was arrested last week for allegedly putting stickers supporting the protests outside his home.

“Many acts which were previously not treated as criminal, especially certain forms of political expression, are now illegal,” said Ryan Mitchell, a legal academic at the Chinese University of Hong Kong.

Fears for the independence of the legal system increased in September last year, when Carrie Lam, the city’s leader, contradicted top judges to say that there was no separation of powers in the territory between the executive, legislature and judiciary.

For Beijing, the reforms to the legal system were justified by the 2019 protests, which showed that the city’s “overall legal order” had been “significantly undermined”, according to Han Zhu, a research professor at the University of Hong Kong.

China believes the territory’s legal system needs to put the country’s national constitution first, rather than Hong Kong’s mini-constitution, the Basic Law, which guarantees rights such as freedom of expression. “The [security law] is surely not the end but the beginning of the legal and political reconstruction process,” Zhu wrote.

Zhou Qiang, president of the Supreme People’s Court of China, reportedly told the Hong Kong judiciary to “fully implement” a Chinese government policy to ensure that every official in the city’s top ranks was a patriot.

The FT revealed last month how pro-Beijing politicians had intervened in the appointment of a top Hong Kong judge — a process that was once largely free from political interference. This has opened up a potential route for Beijing to more directly influence the selection of judges.

For Hong Kong’s judiciary, whose tradition of independence has long attracted multinationals looking to set up their Asia-Pacific headquarters in the city, the real trial may only be just beginning.

“Judicial independence is the key difference between Hong Kong and the mainland . . . and the Hong Kong characteristic most despised by Beijing,” Fisher, the lawyer, said.