The letter, initiated by PEN America, calls on Hong Kong chief executive, Leung Chung-ying to do everything possible “to investigate these cases and demand the release of Cheung Chi-ping, Gui Minhai, Lam Wing-kee, Lee Bo, and Lui Por, who are confirmed to be detained by mainland Chinese police.”

IPA Secretary General José Borghino, who is currently in China for talks with the Publishers Association of China, said: “China’s unhelpful approach to this issue is going to cause further damage to its international reputation, which the country can ill afford. Not only is this a question of Chinese state disdain for free expression, but there is also a clear legal issue at stake. These summary detentions violate the provisions of the 1984 Sino-British Joint Declaration, and Article 28 of Hong Kong’s Basic Law, which protects freedom of expression and press freedom, and protects residents from just this kind of arbitrary arrest.”

Click here for the full text of the letter.