The university had warned any advocacy of independence would be a breach of the Basic Law, which states that Hong Kong is an inalienable part of China.

On late Tuesday night, the student union issued the deadline to outgoing vice chancellor Professor Joseph Sung Jao-yiu and his would-be successor Rocky Tuan Sung-chi to respond further and state their positions on the matter, threatening “escalating action” if they failed to do so.

At least three large black banners bearing the words “Hong Kong independence” in Chinese and English were taken down on the first day of the school year on Monday.

But the stunt was repeated on Tuesday morning at a different location, with a black banner hung in the middle of Cultural Square, an open space near the canteens and bustling with students and staff.

Posters which read “Fight for Our Homeland. Fight for Hong Kong Independence” also filled a nearby “democracy wall”, which allows students to post their ideas freely without prior permission.

Union president Au Tsz-ho said anyone could put up posters on the democracy wall without prior permission, but applications must be made to the union to display banners.

Asked whether anyone had applied to display the controversial banner on Tuesday, he said the union only spotted it in the morning.

“A student had come to us [on Monday] night to retrieve the banner that was previously taken down, but we do not know whether today’s was also his work,” he said.

The school’s Office of Student Affairs stepped in to urge the union to prevent a recurrence.

“We have noted the posters and banners advocating Hong Kong independence. Such ideas have constituted a breach of relevant provisions of the law, and go against the university’s stance of absolute disagreement to independence,” a letter addressed to the student union said.

It also urged the union to strictly enforce the regulations on managing the area – without specifying them – and warned that the office would intervene and take down “inappropriate” materials if necessary.

But Eric Cheung Tat-ming, principal lecturer at Hong Kong University’s law faculty said he could not see any piece of criminal law which could be enforced in this case.

“Of course, we cannot say it was wrong to cite the Basic Law, but I doubt independence advocates would violate criminal ordinances,” he said.

In a written response, the union reiterated Cultural Square's significance in allowing students to express their ideas and discuss current issues.

“It has always been such a place, and should remain unchanged ... Yet the university management has removed a banner without permission from the student union on the commencement day of the academic year,” the statement read.

“We are of the opinion that the action taken by the university is not appropriate, and we would like to express our deep regret at such action. We hereby urge the university to respect the autonomy of the student union, especially in terms of the management of the Cultural Square.”

After a meeting with Office of Student Affairs director Raymond Leung Yu-chiu on Tuesday, the union issued the statement demanding a clarification from Sung and Tuan, and reiterated that it would respect the autonomy and freedom of speech of students.

The student body was coy on whether it endorsed the advocacy of independence.

In a speech at the convocation ceremony on Monday, Au said he was glad to see that students were “doing their best to voice their opinions on matters they believe are right” after spotting such materials in campus.

Sung also said on Monday students were free to express their opinions as long as they did not break the law.

On Wednesday the student union said it would hold a forum to discuss freedom of speech and whether Hong Kong independence should be raised on the campus.

The forum will be held from 12.30pm to 2pm at Cultural Square.

Additional reporting by Elizabeth Cheung