Hong Kong (AFP) - A string of figures critical of China's Communist Party say they have been denied visas to Hong Kong, sparking accusations of a secret "blacklist" that bears signs of Beijing's growing influence over the city.

Taiwanese scholars Wu Rwei-ren and Wu Jieh-min of Taipei's prestigious Academia Sinica, due to speak at a conference in Hong Kong Friday, were the latest to say their visa applications had been rejected without explanation.

Their cases followed a high-profile incident in October when British activist Benedict Rogers, deputy chairman of the governing Conservative Party's human rights commission, was turned away by immigration after landing for what he said was a private trip.

Rogers has previously criticised the jailing of democracy campaigners in Hong Kong, calling it "one of the most grotesque miscarriages of justice I have seen."

Semi-autonomous Hong Kong enjoys freedoms unseen on the mainland since being handed back to China by Britain in 1997, under a "one country, two systems" deal.

But there are rising concerns that those liberties are under threat.

The city's mini-constitution, the Basic Law, stipulates immigration affairs are managed internally, but analysts say decisions over who is allowed in are increasingly arbitrary and non-transparent, sparking concern that immigration is becoming a political battleground.

"Things that were not considered a threat are now seen as threats," said Edmund Cheng, assistant professor of government and international studies at Hong Kong Baptist University.

In a controversial response to media questions over the Rogers case, city leader Carrie Lam refused to provide details about why he was refused entry but suggested that matters of immigration can involve diplomatic considerations, for which Beijing is responsible.

"This redefines some of the boundaries in 'one country, two systems'," said Cheng of Lam's comments.

China banned a cross-party delegation of British MPs from entering Hong Kong in 2014, at the height of the mass pro-democracy Umbrella Movement rallies.

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That visit was part of a foreign affairs committee examination of the former colony's relations with the United Kingdom.

Pro-democracy lawmaker Claudia Mo said she believed Hong Kong was "giving up" its own authority on immigration issues.

"The whole thing was unthinkable 10 years ago, but now it's happening," Mo told AFP.

- Academics targeted -

Rejected Taiwanese scholar Wu Rwei-ren accused the Hong Kong government of having a "blacklist" which was expanding to academics.

Lawmakers from Taiwan's ruling Beijing-sceptic Democratic Progressive Party (DPP) and the island's leading anti-China activists have also previously been rejected from entering Hong Kong.

"Taiwan's academics have actively participated in social movements," Wu told AFP.

He believes Beijing wants to block communication between Taiwanese groups and opposition forces in Hong Kong "to isolate its civil society and render it helpless".

Wu is co-author of a book on Hong Kong nationalism and took part in Taiwan's Sunflower Movement in 2014, which saw a dramatic occupation of parliament to protest a China trade pact.

He said he could not get an entry permit when he applied online to attend a seminar in Hong Kong this month, although he had previously been permitted to visit the city.

The other Taiwanese scholar rejected this month, Wu Jieh-min, is also critical of Beijing and Hong Kong authorities, and participated in various social movements.

Taiwan is a self-ruling democracy with a fully-fledged civil society, but Beijing sees it as part of its territory to be reunified.

Ties have become increasingly frosty since DPP leader Tsai Ing-wen took office as president last year.

Taiwanese academics, campaigners and lawmakers have consistently declared public support for Hong Kong's pro-democracy camp, and the city's high-profile young campaigners frequently visit the island.

Meanwhile Chinese authorities have issued warnings about challenges to its sovereignty, particularly incensed by an emerging independence movement in Hong Kong.

Hong Kong's immigration department said it could not comment on individual cases when asked about the Taiwanese scholars recently rejected, saying only that it would "consider all the factors" when assessing an application.

The department did not respond specifically to queries about allegations of a blacklist.

Taiwan's Mainland Affairs Council, which deals with cross-strait relations, expressed "regret and dissatisfaction" over the barring of its citizens.

Taiwanese social activist and DPP supporter T.C Chang, who also says he was denied entry at Hong Kong's airport this month on his way to a cultural forum, recalls how he felt "honoured" to edit a magazine in the city in the past and had frequently gone back and forth since.

"It's saddening if I can't go to Hong Kong anymore," he said in a Facebook post, but added he would not be stopped.

"My connection with Hong Kong won't be cut off by you."