“Some people publicly called for the city’s independence, while others spread ideas such as ‘self-determination for the nation’, [these ideas] are pro-independence in their nature... and the advocates had publicly ­challenged the country’s constitution,” he said.

“Some people said the ­National People’s Congress should restrain itself from using all its power, but we say that power has to be used, this is our responsibility ... when Hong Kong’s fundamental interest is in jeopardy, and when the baselines of ‘one country, two systems’ are at stake, the NPCSC has to exercise its power.”

While Hong Kong scholars warned the ruling was opening a can of worms with more legal uncertainties, Beijing legal experts said the text was legally sound and showed the determination to oppose any hint of a separatist movement.

The article states that public officers, including lawmakers and principal officials, must swear to uphold the Basic Law and swear allegiance to Hong Kong as part of China “in accordance with law”.

Controversy erupted last month after two pro-independence lawmakers, Sixtus Baggio Leung Chung-hang and Yau Wai-ching of Youngspiration, pledged allegiance to a “Hong Kong nation” and referred to China as “Chee-na”, mimicking the derogatory Shina used by Japan during the second world war. They also displayed a banner saying “Hong Kong is not China”.

Underscoring China’s anger over their antics, Li spent seven minutes explaining why he found their conduct deplorable. He quoted from the book History of Hong Kong, written by a foreign historian and translated into ­Chinese which detailed the ­atrocities inflicted by the Japanese, ­including the raping and killing of thousands.

“There could still be witnesses of these atrocities in the city, and their descendants know what happened,” he said, asking whether those who betrayed and sought to separate their nation in entering the Legislative Council deserved any support.

“I think all Chinese in the country and around the world would support what I said,” Li added.

The interpretation makes it clear that upholding the Basic Law and pledging allegiance to Hong Kong as part of China “are not only the legal content which must be included in the oath … but also the legal requirements” for running for and taking up of the public office concerned.

It then sets out a fourfold meaning of the phrase “swearing- in accordance with law”. First, it means no public ­office or corresponding entitlements shall be assumed or ­enjoyed by anyone who fails to take the oath lawfully.

Second, a person must take the oath “sincerely and ­solemnly”, and “accurately and completely” read out its wording. Third, the oath-taker is ­disqualified from assuming office if they decline to take it properly.

Finally, if the administrator of the oath, such as the Legco secretary general, invalidates an oath, no retaking of the oath is allowed.

The final paragraph makes clear the oath is a “legal pledge made by the public officers specified in the Article to the People’s Republic of China and its Hong Kong Special Administrative ­Region, and is legally binding”.

This marks the first time the law on oath-taking has spelled out the legal obligations categorically towards China first, and on its own, and then Hong Kong.

The paragraph also suggests lawmakers shall be punished for advocating independence, as it reads: “An oath taker who makes a false oath, or …after taking the oath, engages in conduct in breach of the oath, shall bear legal responsibility.”

Asked whether the interpretation would set the precedent for further moves on city affairs, Li said Hong Kong people “can trust the NPC”.

“We would not interfere with issues within the city’s scope of autonomy … we only interpret laws when necessary,” he said.

A Hong Kong court has yet to deliver its ruling on a judicial ­review filed by the government to disqualify Leung and Yau, amid legal sector concerns about Beijing pre-empting the judgment.

But Li dismissed such worries. He said when a disagreement in Hong Kong regarding the mini-constitution “affects the correct implementation of the Basic Law and the principle of ‘one country two systems’, the NPCSC has the responsibility to explain the Basic Law in a timely manner”.

“This safeguards the rule of law, and a manifestation of the spirit of the rule of law,” he said.

Li also said the oath taking controversy showed “loopholes” in Hong Kong laws should be plugged.

Asked whether localist lawmaker Lau Siu-lai may face ­disqualification for stopping several seconds between every word in her oath-taking last month, Li would only say: “Interpretations are explaining the legislative intent of an article, it is not a new legislation in itself … Its validity is included in the validity of the law that it seeks to explain.”

On the point that lawmakers should “bear legal responsibility” for making false vows, Peking University legal professor Rao Geping, a member of the Basic Law Committee, said it meant lawmaker could face criminal investigation. Citing Article 79, Rao said a lawmaker could be disqualified if two-thirds of lawmakers passed a motion to condemn them.

Separately, another committee member, Albert Chen Hung-yee, said Beijing was adopting a “strict” approach.

But Johannes Chan Man-mun, a constitutional law specialist at the University of Hong Kong, said the decision amounted to “giving instructions” to a judge as to how to rule in a case, hurting ­judicial independence. “By adding the requirement for ‘sincere’ oath-taking, the ruling is opening a can of worms,” Chan said. “In validating the oath, should the administrator vet every lawmaker and candidate like checking their Facebook remarks?”

Another constitutional law professor, Benny Tai Yiu-ting, said the ruling “amended” the Basic Law by adding one more condition for disqualifying lawmakers to the seven listed in Article 79, which says a lawmaker committing misbehaviour or breach of oath will be disqualified on a two-thirds majority by his colleagues.

Democratic camp convenor James To Kun-sun said Beijing was interpreting a local ordinance rather than the Basic Law, which appeared to him an “illegal structure” to the mini-constitution.