The imprisonment of three prominent Hong Kong democracy activists earlier this year is a “serious threat” to the rule of law and violates international treaties, a group of leading attorneys has said, amid claims the city’s courts are increasingly politicised.

In the open letter, the lawyers, including former lord chancellor Charles Falconer and six other Queen’s Counsel, heavily criticised the jailing of Joshua Wong, Alex Chow and Nathan Law in August for their role in pro-democracy protests.

The trio avoided jail terms last year when they were convicted of “illegal assembly” and sentenced to community service under Hong Kong’s public order ordinance. The government appealed to have the sentences reviewed, which resulted in stiffer penalties of six to eight months in prison.

“The independence of the judiciary, a pillar of Hong Kong, risks becoming a charade, at the beck and call of the Chinese Communist party,” the letter said.

“Hong Kong’s rule of law and basic freedoms, at the heart of the principle of ‘one country, two systems’, now face grave threats,” it continued, referring to the arrangement negotiated between the UK and China to grant Hong Kong autonomy.

The letter was signed by 11 prominent lawyers, including Jared Genser, attorney for Chinese Nobel peace prize laureate Liu Xiaobo, Kirsty Brimelow QC, chair of the Bar Human Rights Committee of England and Wales, and Sir Geoffrey Nice QC, former chief prosecutor in the trial of Slobodan Milošević.

Alex Chow is one of the activists to have been jailed. Photograph: Tom Phillips/The Guardian

The letter also comes days after British human rights activist Benedict Rogers was barred from entering Hong Kong without explanation, and the city’s leader warned others could be banned in the future at Beijing’s whim.

Writing in the Guardian after the incident, Rogers voiced many of the same concerns.

“I have become increasingly concerned about the erosion of Hong Kong’s freedoms and the rule of law, and the threats to the ‘one country, two systems’ pledge,” he said.

The lawyers also heavily criticise the law used to jail the democracy campaigners, the public order ordinance, saying it went against the international covenant on civil and political rights. The United Nations has previously said the law places “excessive restrictions” on fundamental rights.

The jailings represented “a serious threat to the rule of law and a breach of the principle of ‘double jeopardy’ in Hong Kong”, the letter said.

The harsher sentences were necessary because of a “sick trend” of anti-government protests, the judge argued.

Last week 20 activists, including Wong, were convicted of criminal contempt of court in connection with the same democracy protests, although they have yet to be sentenced.