Benedict Rogers, deputy chair of the party's human rights commission, was escorted onto a plane back to Thailand, where he had come from, according to his lawyer.
Hong Kong lawyer Albert Ho, who heads the Hong Kong Alliance in Support of the Patriotic Democratic Movement in China, said he had tried to meet with Rogers after being instructed as his lawyer, but hadn't been able to speak to any immigration officials until after he had left.
"He called me to say that he was at the immigration counter at the airport, being taken into a side-room with a high-ranking immigration officer," Ho told RFA. "Then they said they were refusing him entry."
"They kept asking him what was the purpose of his visit to Hong Kong, and he kept telling them it was a private visit, but they still refused to let him in."
While the city's chief executive Carrie Lam declined to comment on individual cases, Rogers was quoted in Hong Kong's South China Morning Post as saying he had been warned informally by Chinese officials that he wouldn't be allowed to enter the former British colony, which has maintained its own immigration controls since its handover to China in 1997.
Rogers has previously voiced concerns that promises of a "high degree of autonomy" and the "one country, two systems" principle promised by China before the handover may now have been compromised.
Rogers told the SCMP that he had had a number of indirect warnings from the Chinese embassy in London, amid official fears that he intended to visit the jailed student leaders of the 2014 democracy movement, Joshua Wong, Nathan Law and Alex Chow.
Officials had told him his visit would be "a grave threat to Sino-British relations," although he had assured them he had no such plans.
Rogers has previously met with other Hong Kong democracy activists, including Eddie Chu, Derek Lam and Raymond Chan.
But he said he felt it necessary to put the "one country, two systems" principle to the test, rather than "caving in" to unofficial warnings via text message.
Ho said he had been deliberately prevented from seeing Rogers before his departure.
"I think I must have called them eight times, but they deliberately weren't taking my calls, because they know my number," Ho said. "They knew it was me calling."
"They kept me waiting for an hour before they told me he had already left Hong Kong, so I couldn't meet with him," he said. "It's disgusting that they would treat a person's lawyer that way; they pay no attention to the rule of law."
Democratic Party lawmaker Ted Hui said he believes the decision was entirely politically motivated.
"I genuinely believe that they did this for political reasons, but they won't answer such questions," Hui said, adding that a judicial review might cast some light on the processes leading up to Rogers' removal.
Meanwhile, Ho said the immigration department is likely feeling the impact of China's massive "stability maintenance" operation in the run-up to the ruling Chinese Communist Party's 19th party congress on Oct. 18.
"The state security police and the regular police in mainland China take direct control over the Hong Kong immigration status of certain politically sensitive people," Ho said, adding that such an influence is in breach of Hong Kong's mini-constitution, the Basic Law.
"The Basic Law states very clearly that immigration decisions are for the head of our immigration service to make, but I have reason to believe that this isn't actually the case at all."
Hong Kong's Immigration Ordinance states that officers can refuse entry to Hong Kong after the examination of any visitor.
But Alvin Yeung, who leads the pro-democracy Civic Party, said many people are now concerned about being denied entry to Hong Kong on the say-so of Beijing, rather than the city's own immigration rules.
"The question we must ask is this: on what grounds did the immigration department refuse entry to Rogers?" Yeung said.
"Also, was this the decision of the immigration department, or the result of orders from some external agency or higher up?"
In London, British foreign secretary Boris Johnson said the decision merited an explanation from Beijing.
"I am very concerned that a U.K. national has been denied entry to Hong Kong," Johnson said in a statement. "The British government will be seeking an urgent explanation from the Hong Kong authorities and from the Chinese government."
"Hong Kong's high degree of autonomy, and its rights and freedoms, are central to its way of life and should be fully respected," he said.
In February 2016, the U.K. accused Beijing of breaching the handover treaty by removing bookseller and British national Lee Bo, whose "disappearance" alongside four colleagues has been linked to the plans to publish a book about the Chinese president.
In a six-monthly statement to parliament on the implementation of the 1984 Sino-British Joint Declaration under which Hong Kong was handed back to China in 1997, then foreign secretary Philip Hammond said Lee was "involuntarily removed" across the internal immigration border to mainland China.
Causeway Bay Books store manager Lee Bo, 65, was last seen at work on Dec. 30, 2015, after four of his associates, publisher Gui Minhai, general manager Lui Bo, and colleagues Cheung Chi-ping and Lam Wing-kei had already gone missing.
There is no record of Lee leaving Hong Kong, suggesting that he was spirited across the internal immigration border by Chinese police, while Gui, who holds a Swedish passport, was apparently kidnapped while on vacation in Thailand.
Gui's daughter Angela told RFA on Wednesday that her father is scheduled for release at the beginning of next week at the end of a two-year jail-term for his role in an alleged "drunk driving" death.
"I hope they will [release him], but I have a feeling there is another trial in the pipeline," she said. "I think there is still a charge outstanding that they will try him for, but I hope that they will let him go."
Reported by Wong Lok-to and Hai Nan for RFA's Cantonese Service, and by Gao Feng for the Mandarin Service. Translated and edited by Luisetta Mudie.
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