Taiwan's semi-official Central News Agency reported that Lam had died at Mackay Memorial Hospital in Taipei after a battle with cancer, citing an unnamed source.

"My friend Lam Wing Kee of #CausewayBayBooks died of lung cancer in Taipei today," former Hong Kong pro-democracy lawmaker Claudia Mo said in a social media post.

Last year, Lam told the media that his lung adenocarcinoma had recurred and had progressed to stage four. Earlier this month, he closed his bookshop, citing health reasons.

He was one of five publishers selling gossip-filled tomes on China's leaders who vanished at the end of 2015, resurfacing in mainland custody and making televised confessions.

Their disappearance caused widespread alarm in Hong Kong.

Lam was allowed back into Hong Kong in June 2016 on condition that he pick up a hard drive listing the bookstore's customers and return to the mainland.

But he instead skipped bail and went public with explosive testimony detailing how he was blindfolded by mainland police after crossing the border at Shenzhen and spent months being interrogated.

In April 2019, Lam fled to Taiwan after Hong Kong announced plans to allow extraditions to China, a move which sparked months of huge and sometimes violent democracy protests in the financial hub later that year.

He told AFP after he had flown to Taipei that "right now, Hong Kong is not safe for me anymore", adding that he was "enjoying the air of freedom".

A year later, the dissident reopened his store Causeway Bay Books in Taiwan, where he described the self-governed island as "a place with freedom and democracy and we still have the right to read books".

Taiwan's president Lai Ching-te said Thursday he was "deeply saddened" about Lam's death, extending condolences to Lam's family and friends.

Lam's life "bore witness to the value of freedom of expression, and to the fear and suffering inflicted by authoritarian repression", Lai said in a Facebook post.

"He chose not to remain silent. Instead, he reopened Causeway Bay Books in Taiwan, turning it into a place where friends from Hong Kong could gather, speak out and support one another," he added.

"Taiwan will continue to stand firm in defending democracy, freedom and human rights, and stand with all those who refuse to bow to authoritarianism."