Ma Ying-jeou was due to give a speech at a Society of Publishers in Asia Media Awards ceremony on Wednesday
Taiwan's opposition Nationalist Party slammed the self-ruled island's new government as "suppressive" for barring former president Ma Ying-jeou from travelling to Chinese-controlled Hong Kong on the grounds of national security.
Ma's China-friendly Nationalists lost landslide elections in January to President Tsai Ing-wen and her Democratic Progressive Party (DPP), which has traditionally leant towards independence from China.
They said barring Ma from visiting Hong Kong was "completely unhelpful for Taiwan's internal political reconciliation and social harmony".
"This suppressive style of the just-sworn-in DPP government is naked to all the people," it said.
Tsai's administration said on Sunday it had barred Ma from travelling to Hong Kong where he was due to give a speech at a Society of Publishers in Asia Media Awards ceremony on Wednesday.
"Based on the international situation faced by Taiwan and in consideration of national security and interests, Hong Kong is a highly sensitive area (when it comes to) maintaining our national security," the Presidential Office said in a statement, adding that it would be difficult to control the risks of such a visit.
It also said Taiwan and Hong Kong authorities had had no time to coordinate on preparations.
The DPP has traditionally been more wary of China than Ma's Nationalists. Its caution comes after a group of Taiwanese were forcibly deported from Kenya to China for suspected fraud in China in April, a case that enraged Taiwan.
Some comments on Taiwan social media at the time questioned whether a precedent was being set of Taiwanese abroad being "taken away" by China, drawing a parallel with the case of five booksellers in Hong Kong who temporarily went missing in mysterious circumstances.
The Society of Publishers in Asia said in a statement on its website it was disappointed Ma would not be able to attend but that it planned for him to speak via a video link.
China has claimed sovereignty over Taiwan since 1949 when Mao Zedong's forces won the Chinese civil war and Chiang Kai-shek's Nationalists fled to the island. Communist Party rulers in Beijing have vowed to bring Taiwan under its rule, by force if necessary.
Hong Kong returned to Chinese rule in 1997 under a "one country, two systems" formula which guarantees wide-ranging autonomy for the financial hub but was the scene of lengthy and sometimes violent anti-China street protests in 2014 calling for fully democratic elections.
China had put forward the "one country, two systems" formula as a model for Taiwan to follow, which island leaders rejected.
China has also repeatedly warned Taiwan of negative consequences if they fail to recognise Taiwan is a part of China under Beijing's "one China" principle.
Tsai has said she will maintain the status quo with China but has not repeated the "one China" principle.