TEHRAN, Young Journalists Club (YJC) - Agnes Chow, 21, a former leader of the mass Umbrella Movement protests of 2014 that called for political reform, had her nomination rejected because she supports self-determination for the semi-autonomous city, the government said.
It comes as fears grow that political debate in the city is being shut down under pressure from an assertive Beijing, with the recent jailing of democracy activists fuelling concern.
The emergence of campaigners calling for independence for Hong Kong since the failure of the Umbrella Movement to win reform has incensed Beijing, and President Xi Jinping has made it clear that he will not tolerate any challenge to Chinese sovereignty.
The pro-Beijing Hong Kong government has previously barred independence activists from standing for office, but Chow's ban is the first against a more moderate campaigner.
She had been hoping to stand in by-elections in March, which were triggered by the disqualification from the legislature of six lawmakers who protested while taking their oaths of office in 2016.
"Self-determination or changing the Hong Kong Special Administrative Region (HKSAR) system by referendum which includes the choice of independence is inconsistent with the constitutional and legal status of HKSAR," the government said.
It added that someone who "advocates or promotes" self-determination or independence cannot uphold the city's mini-constitution, the Basic Law.
Chow is a member of Demosisto, a political party co-founded by leading democracy campaigner Joshua Wong, who is currently out on bail after being jailed for his role in the 2014 rallies.
Demosisto does not campaign for independence but advocates self-determination and a referendum for Hong Kong people to decide how they want to be governed.
The ban on Chow has wide-scale implications for other similar activists wanting to stand for office, including Wong.
Demosisto said it "strongly condemns" what it called a political decision.
"The government's motivation is to eliminate the hopes of an entire generation of young people," it said in a statement.
The ban is also another set-back for the pan-democratic camp, which is trying to win back the six seats it lost due to the disqualifications.
Losing the six seats robbed it of the one-third minority vote needed to block important bills in the pro-Beijing legislature.
Hong Kong was handed back to China in 1997 under a "one country, two systems" deal that grants it a partially elected legislature and rights unseen on the mainland, including freedom of speech and the right to protest.
But there are growing fears those liberties are being eroded.
The Basic Law specifies that Hong Kong is an "inalienable" part of China, but activists say that does not mean views that challenge that status should be silenced, given that freedom of speech is protected.