“As the chief executive, who is accountable to the central government, I have to submit reports from time to time … that is only legitimate,” she said.

Last week, Lam and her advisers in the Executive Council upheld the ban on the HKNP, backing the security minister’s decision last September to outlaw the party for posing a threat to national security and public order. The party now has three months to decide whether to seek a judicial review.

The letter that followed from the central government, as recited by Lam and later published by Chinese state news agency Xinhua, expressed support for her administration’s decision.

While reiterating the city’s responsibility to safeguard national security, Beijing also underscored the fact Lam and her administration were directly accountable to both the central government and the city.

Beijing loyalists in the city agreed it was rare for such a letter to the chief executive to be made public, but insisted it only showed the central government’s concern for national security. But opposition lawmakers questioned whether Beijing had infringed on the city’s autonomy.

The chief executive has in the past submitted annual reports to Beijing, but they have never been publicly disclosed. The government has turned down media requests for copies of such reports, even though they were made under a public disclosure code.

Lam added she was inclined to make her report public, pending legal advice from the justice department.

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Maria Tam Wai-chu, vice-chairwoman of the Basic Law Committee, which advises the National People’s Congress on matters relating to Hong Kong’s mini-constitution, said the unprecedented move by Beijing underlined its zero tolerance of separatists.

“As far as I can recall, it is the first time Beijing has asked Hong Kong to do something through an official letter,” Tam said. “It’s a show of consent and support, not criticism or exerting pressure, as the government has completed its process on the case.”

Lau Siu-kai, a former head of the Central Policy Unit, the Hong Kong government’s think tank, also agreed such public requests were rare, although Beijing has in the past asked Hong Kong for an account of notable political events.

“The idea is to convey Beijing’s position to all, including the courts,” said Lau, who is now the deputy head of a semi-official mainland think tank, The Chinese Association of Hong Kong and Macau Studies.

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A government source, however, noted that the local government had been submitting such reports to Beijing on major incidents, including accidents that resulted in serious casualties.

“The chief executive wanted to publicise the report so members of public would not find it ‘secretive’. [The report would help] Beijing understand the process, facts and procedure of the prohibition and not rely only on media reports,” the source said.

The report on the ban was expected to be filed very soon, the source said, as it would only include facts which had already been made public.

Questioning the need for a report with so much information already in the public domain, Democratic Party chairman Wu Chi-wai said the request showed Beijing’s lack of faith in the city’s governance, and questioned whether the move infringed on the city’s high degree of autonomy.

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The opposition Civic Party also accused Beijing of meddling in Hong Kong’s internal affairs.

Political commentator Johnny Lau Yui-siu said Lam’s public announcement on Tuesday might have been required by Beijing, as a way of reiterating its hardline stance against the pro-independence movement in Hong Kong.

“They want to scare people from talking about independence, but it may not work,” Lau said.

Additional reporting by Jun Mai