“The three steps looped [together] form the legal basis [of the plan],” Tam said.

“If someone in Hong Kong mounts a challenge, there has to be the most authoritative figure saying this is in line with the Basic Law. That decision provides a basis for formulating local legislation.

“Without this decision, the local legislation would certainly be overturned …”

She added: “The binding power does not tell you directly what you can or cannot do. The binding power is about whether someone challenges the law formulated by our Legislative Council or the conduct of our government. The courts, I believe, will accept the NPC’s claim that [the decision] is in line with the Basic Law … That’s the binding power.”

Six things to know about Hong Kong’s controversial ‘co-location’ joint checkpoint scheme

Tam’s latest remarks came after fellow Basic Law Committee members Elsie Leung Oi-sie and Albert Chen Hung-yee offered their views on the binding power of the NPCSC’s decision.

Leung, the city’s first secretary for justice after it returned to Chinese rule in 1997, said she “completely agreed” the decision had no binding power while Chen, a law professor at the University of Hong Kong, believed the relevant laws were more complicated and said the issue “belonged in a grey area”.

But Leung had said that China’s constitution empowered the NPCSC to supervise decisions made by China’s cabinet, the State Council, including the cooperation agreement signed between the Hong Kong and Guangdong governments in November on the joint checkpoint plan.

Chen also dismissed fears that the checkpoint plan would set a precedent to undermine Hong Kong’s autonomy, saying the city has control in two key parts of the three-step process.

Newly approved joint checkpoint plan will damage Hong Kong’s rule of law, legal heavyweights say

Pro-establishment lawmaker Priscilla Leung Mei-fun, a barrister by profession, agreed on Monday with Tam’s earlier assessment and explained that under the system of Chinese constitutional laws, the NPCSC’s decisions possess the same binding powers as its interpretations.

“It’s always been like that,” she said on the same programme.

Hong Kong leader Carrie Lam Cheng Yuet-ngor said a three-step process by which Hong Kong would implement the arrangement for the line to Guangzhou was “entirely prudent and lawful”.

However, Civic Party lawmaker Tanya Chan, a barrister and convenor of a concern group on the co-location arrangement, stressed it was the government’s responsibility to provide a clear account.

When asked if she would rather see an interpretation, Chan replied: “To be honest, if [the interpretation] had followed all procedures … I would have understood.”

Professor Johannes Chan Man-mun, a former law dean at the University of Hong Kong, said he and others in the legal profession were only seeking an objective and reasonable explanation on the legality of the NPCSC’s decision, not to challenge the decision in itself.

The academic also said that even the highest organ of state would have to be subjected to restraints and cannot consider one’s words final or that would turn into “rule of man”.

Also on Monday, MTR Corporation chairman Frederick Ma Si-hang revealed that construction of the project was 98 per cent complete and they were preparing to commence operations in September.

“Of course, whether we can commence service depends on the joint checkpoint plan. We have no Plan B, we cannot operate if the plan is voted down,” he said.

“I’m not in the legal sector, but having read the government’s statement, I consider its statement well-founded. The three steps are very transparent. They have to pass the local legislation.”

Ma urged the parties to set aside their disagreements for the convenient measure of co-location to come to fruition.

“Without the plan, the express rail link would simply be a faster express rail link. There would be no meaning to it,” he said.

“If we cannot connect to the world’s largest express rail link, I think it would be Hongkongers’ loss.”