Hong Kong's police have thrown out an organiser's request to hold the July 1 march, while a separate appeal against the rejection of a similar application was thrown out.
The refusal to give the green light for the demonstration organised by the Civil Human Rights Front - a coalition of pro-democracy groups and activists - is the first since 1997 when Britain handed the territory back to China.
The annual pro-democracy march, which starts in Causeway Bay and ends in Admiralty, could have been the first large protest held after the controversial national security law kicks in. China resumed sovereignty over Hong Kong, after more than a century of British colonial rule, on July 1, 1997.
The new national security legislation mooted by Beijing is widely expected to be enacted by Tuesday - the eve of the handover anniversary - when China's top legislative body meets.
The Front released the police rejection letter yesterday. The letter cited health safety as a key reason for dismissing the request, noting that the coronavirus is highly infectious, and urging residents to maintain social distancing.
The police said: "The public assembly and the march are highly dangerous activities, so the police have reasons to believe such activities would... be a threat to the public... and affect the rights of people."
They added that since last June, the Front has held eight demonstrations that have either ended in violence or had unrest break out.
Last year, the July 1 march, held amid the anti-extradition Bill movement, was said by organisers to have attracted a crowd of 550,000.
The Front's vice-convenor Eric Lai had expected the ban. "It is certainly a political decision by the authority," he told The Sunday Times, adding that the Front would appeal against the decision.
At the same time, an appeal board yesterday dismissed a district councillor's attempt to overturn the police ban on rallies he had hoped to hold today and on Wednesday.
Eastern district councillor Andy Chui had wanted to stage a march from Causeway Bay to Chater Road in Central on the two days. Similarly, police cited health worries and possibility of violence.
The crowd last year for the march, held amid the anti-extradition Bill movement, according to organisers.
Both the Front and Mr Chui said the police's reasons for refusal are inconsistent as theme parks which attract huge crowds - such as Ocean Park - can reopen.
Hong Kong's sole member of the National People's Congress Standing Committee, Mr Tam Yiu Chung, said yesterday that he will flag to Beijing calls for the law to be retroactive and for the penalties to be "heavy enough".
He added that it was "meaningless" to bring up some people's opposition to the impending legislation at the Standing Committee meeting.