Li Yanhe, editor-in-chief of Gusa Publishing, went missing during a visit to mainland China in 2023 before Chinese authorities revealed he was being investigated for national security crimes.

On Wednesday, Beijing's Taiwan Affairs Office (TAO) -- responsible for relations with the island -- said Lee had been tried and sentenced in Shanghai in February.

Spokesman Chen Binhua told a press conference that Li pleaded guilty and had not appealed the judgment.

Mainland-born Li lived in Taiwan, where he had published books critical of China's ruling Communist Party, including a history of alleged oppression in the western region of Xinjiang.

China has ramped up military and political pressure on self-ruled Taiwan in recent years, which it claims as its territory.

Beijing has threatened to use force to bring the island under its control.

This month, Taiwanese President Lai Ching-te branded China a "foreign hostile force" and proposed measures to combat Chinese espionage and infiltration.

Beijing also on Wednesday announced a new reporting portal for people to inform on "malicious acts" by Taiwan independence "thugs and accomplices".

The TAO's website homepage now features a floating pop-up box for a "reporting special column", which leads users to another page with an email address for "victims of persecution" to contact.

Announcing the new email inbox, spokesman Chen accused Taiwan's ruling Democratic Progressive Party (DPP) of "fabricating charges to suppress different opinions on the island, shutting down freedom of speech... and suppressing" advocates for developing relations with China.

The informant's inbox comes after Taiwan revoked the residency permit of a mainland Chinese influencer who had advocated unification by military means.

Taiwan's National Immigration Agency investigated Liu Zhenya after she posted on Douyin, the Chinese version of TikTok, in support of Beijing's unification with Taiwan by force.

She had lived on the island for over ten years but left on Tuesday after a court rejected her petition against a government deportation order.