The three other legislators who were unseated, Leung Kwok-hung, Lau Siu-lai and Edward Yiu, had delivered their oaths with various displays of defiance, including by reading it extremely slowly, inserting words calling for democracy and displaying props.
The Department of Justice of Hong Kong SAR government submitted an application for a judicial review to the High Court on December 2, 2016, requesting the court to rule whether the four LegCo members' oaths comply with the Hong Kong Basic Law and whether they are still qualified to serve.
High Court Judge Thomas Au said today in a written judgement that the Basic Law interpretation was "binding" and that "the word "solemn" bears the commonly understood meaning of being dignified and formal".
The unprecedented intervention was prompted by a string of protests during the swearing in of lawmakers the month before.
Concerns China is squeezing Hong Kong have sparked calls by some activists for self-determination or even independence for the city, angering Beijing.
Two pro-independence activists were ousted from the chamber in November after China's top legislature intervened during their hearings to clarify that officials were required to take their oaths "solemnly and sincerely".
Law's party Demosisto condemned "the manifest interference of the Beijing government to cripple Hong Kong's legislative power".More news: Trump Won't Say Whether He Believes Putin Denials on Election Meddling
Among the four disqualified on Friday was Nathan Law, 24, the youngest person to win in a seat and a leader of the "Umbrella movement" protests in 2014.
"Suppression is not scary", he told reporters.
Campaign group Human Rights Watch described the judgement as a "another alarming blow" to Hong Kong's autonomy.
The handover agreement enshrined liberties unseen on the mainland, including freedom of speech and an independent judiciary, but Beijing has been accused of trampling the deal.
Before taking his oath, Nathan Law quoted Mahatma Gandhi, saying that "you can never imprison my mind".
Law was disqualified for adding words to his oath and adopting a tone of voice that "expressed a doubt on or disrespect of the status of the PRC as a legitimate sovereign of the Hong Kong Special Administrative Region", the court said in a summary of its ruling, referring to the People's Republic of China. "We might have to fire some people", he said.