(L to R) Joshua Wong, Nathan Law, and Alex Chow, Hong Kong protest leaders.

A dozen barristers and judges, including Gibson, Dunn & Crutcher London partner Charlie Falconer, QC, penned an open letter on Sunday criticizing a Hong Kong court’s decision to send three student activists to prison for unlawful assembly.

In August, Joshua Wong, Alex Chow and Nathan Law were sentenced by the Court of Appeal to six-, seven- and eight-month prison terms respectively for their involvement in the Occupy Central pro-democracy protests in 2014.

“As lawyers, we regard the imprisonment of Joshua Wong, Alex Chow and Nathan Law in Hong Kong as a serious threat to the rule of law,” the open letter says.

In addition to Falconer, former Lord Chancellor and Secretary of State for Justice of the U.K. under Prime Minister Tony Blair, the open letter was also signed by 11 international human rights lawyers, including former United Nations chief war prosecutor Desmond Lorenz de Silva, QC, Kirsty Brimelow QC, chair of the Bar Human Rights Committee of England and Wales, Sir Geoffrey Nice QC, former chief prosecutor in the trial of Slobodan Milošević, and U.S. lawyer Jared Genser, attorney for the late Chinese dissident and Nobel Peace Prize laureate Liu Xiaobo and his widow Liu Xia.

The August decision came after the Hong Kong Department of Justice appealed a lighter sentence of community service handed down previously by a lower court. The Court of Appeal decision was subsequently criticized by members of the legal community and by the press for potential breach of the so-called “double jeopardy” principle, which protects defendants from facing criminal prosecution more than once for the same offense.

The open letter also criticizes the Hong Kong Public Order Ordinance—the law used to send the Occupy Central activists to prison.

“The Public Order Ordinance is incompatible with the International Covenant on Civil and Political Rights (ICCPR), which applies to Hong Kong. Human rights organizations have long urged Hong Kong to revise the ordinance to comply with the ICCPR,” the letter says.

Born out of the 1967 riots against British colonel rule, the Public Order Ordinance was most recently amended in 1997, on the eve of the hand over of Hong Kong’s sovereignty from Britain to China. The 1997 amendment, which required protest organizers to obtain government approval, has been controversial, as it allowed the government to restrict citizens’ right to publicly assemble.

The open letter also accuses the Chinese government for its role in eroding the independence of the judiciary in Hong Kong.

“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 states, referring to a 2014 government white paper issued by Beijing stating that the central government has comprehensive jurisdiction over Hong Kong.

In August, responding to rising concern about Hong Kong’s judicial independence coming under threat after the Court of Appeal decision, the Hong Kong Bar Association and the Law Society of Hong Kong released a joint statement.

“The decisions by the Hong Kong courts are made solely according to law upon applications by one party or the other. We see no indication otherwise in respect of the recent cases which have generated widespread comment,” read the statement, referring to the jailing of the student leaders.

“Unfounded comments that judicial decisions were made or influenced by political considerations originating outside Hong Kong are unjustified and damaging to our legal system, and to Hong Kong as a whole,” the statement said.