(Adds government response)
HONG KONG, Dec 29 (Reuters) - An association representing Hong Kong barristers said it was "appalled" by the Chinese parliament's move to enforce mainland laws inside a Hong Kong railway station, denouncing it as the most retrograde step since the 1997 handover.
The Hong Kong Bar Association said the decision would "severely undermine" confidence in the rule of law in the former British colony which returned to Chinese rule 20 years ago with the promise of a high degree of autonomy.
China's parliament on Wednesday said part of a high-speed railway station being built in Hong Kong would be regarded as mainland territory governed by mainland laws under a so-called cooperation agreement.
Chinese and Hong Kong officials both argue such a joint immigration checkpoint is necessary for passengers' convenience.
Hong Kong operates under a "one country, two systems" principle which allows it to run its own police force, immigration controls and an independent, British-style judicial system, with lawyers split into barristers and solicitors.
The city's mini-constitution, the Basic Law, explicitly states national laws, with a few exceptions, do not apply in Hong Kong.
"This plainly amounts to an announcement by the NPCSC (National People's Congress Standing Committee) that the cooperation agreement complies with the constitution and the Basic Law 'just because the NPCSC says so'," the bar association said.
"Such an unprecedented move is the most retrograde step to date in the implementation of the Basic Law, and severely undermines public confidence in 'one country, two systems' and the rule of law.
"The integrity of the Basic Law has now been irreparably breached."
The prospect of Beijing interfering in the financial hub has already stoked social tensions and protests including the 2014 "Occupy" street demonstrations that demanded, in vain, full democracy for the city of 7.3 million.
The abduction of Hong Kong-based booksellers in 2015 who later showed up across the border in Chinese custody also touched a raw nerve.
The high-speed rail station is due to open next autumn in the heart of the city, next to the Victoria Harbour, one of the most famous views in the world.
The Hong Kong government said it respected the rule of law.
"Under each system, there must be the highest and ultimate organ of power (and this must be necessary)," it said in a statement.
"Under 'one country, two systems', Hong Kong enjoys a high degree of autonomy accorded by the Basic Law, but we must also respect the national constitution and the NPCSC's status and authority under the country's constitutional system.
"This decision... has legal effect. It is not, as some comments suggest, simply an administrative decision ... and much less reflects a 'rule of man' situation or a retrograde move in the implementation of the Basic Law."
Elsie Leung, deputy director of the Basic Law Committee in the NPCSC and the city's former legal chief, also disputed the association's comments.
"I think their understanding of the Chinese constitution is insufficient," Leung told reporters in comments carried by Hong Kong's Cable Television.
"If you insist on using only common law principles to interpret the Basic Law, that is wrong."
At a cost of more than HK$84 billion ($11 billion), Hong Kong's Express Rail Link will link up with the rest of China's high-speed rail network.
The plan is expected to sail through Hong Kong's legislature in the first half of next year as opposition lawmakers have lost their veto power after a series of controversial court cases. (Reporting by Venus Wu; Editing by Nick Macfie)
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