A dozen barristers and judges, including Gibson Dunn & Crutcher London partner and former Lord Chancellor Charles Falconer QC, have penned an open letter criticising a Hong Kong court's decision to send three student activists to prison for unlawful assembly.

This August, Joshua Wong, Alex Chow and Nathan Law were given 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," reads the open letter.

In addition to Falconer, former secretary of state for justice under Prime Minister Tony Blair, the letter is also signed by 11 international lawyers including former United Nations chief war prosecutor Desmond Lorenz de Silva QC; Doughty Street's Kirsty Brimelow QC, who chairs the Bar Human Rights Committee of England and Wales; and US lawyer Jared Genser, who has represented late Chinese dissident Liu Xiaobo and his widow Liu Xia.

The sentences handed down this August came after the Hong Kong Department of Justice appealed a lighter sentence of community services imposed by a lower court. The Court of Appeal decision was subsequently criticised by members of the press and the legal community for potential breach of the so-called 'double jeopardy' principle, which protects defendants from facing criminal prosecution more than once for the same offence.

The open letter also criticises the Hong Kong Public Order Ordinance, the law used to send the Occupy 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 organisations have long urged Hong Kong to revise the ordinance to comply with the ICCPR," the letter reads.

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

The open letter also points the finger at 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 central government has comprehensive jurisdiction over Hong Kong.

This August, in response to rising concerns over Hong Kong's judicial independence, 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.