She also said China was compelled to take the step at the national level because opposition in Hong Kong's own legislature and among government critics made it impossible to do so locally. They have also raised fears about the stability of the Hong Kong dollar's HKD=D3 (HKD) 36-year old peg to its US counterpart, prompting local officials to issue several reassurances.
Amid rising tensions, the USA government is also reportedly looking to sell the consular staff quarters it owns in Hong Kong. Analysts say that would increase liquidity in the Hong Kong stock market and improve shares' upsides.
The ambiguity in the United Kingdom government position about the precise extent of its offer to Hong Kong citizens, and the pathway to citizenship, may reflect differences within the cabinet.
But even before China announced its plan for the security law there was a surge of renewals of BNO passports, highlighting what many Hong Kong people see as an urgency to secure a back-up plan.
Though they are British passports that allow a holder to visit Britain for six months, they do not come with an automatic right to live and work there. China and Hong Kong officials have justified the law as necessary to restore order to a city that has been racked by sometimes violent anti-China, anti-government protests over the past year.
There are some 350,000 people living in Hong Kong who now hold British National (Overseas) (BN (O)) passports, while some 2.5 million are eligible to get one.
Pro-democracy activist Joshua Wong, holding a placard, speaks to media to urge European leaders against the national security law for Hong Kong outside the Legislative Council in Hong Kong on Wednesday.
China lodged stern representations with the United Kingdom on Wednesday over its foreign secretary's remarks on Hong Kong, saying Britain has no jurisdiction or supervision over the Chinese city.
The new national security law for Hong Kong, which may come into force in August or September, gives the central government of China more authority to directly deal with terrorism, sedition and other threats in the autonomous city.
BN (O) passports have been issued to permanent residents of the former British colony and their children since 1987, a decade before Hong Kong was transferred back under Beijing's sovereignty.
This "could place them on a route to citizenship", Mr Johnson said.
The Passport Office did not immediately respond to a request for comment. They have prospered hand in hand with China's economic renaissance; today their home is one of the richest cities in the world and hundreds of mainland companies have chosen to list on the Hong Kong stock exchange.
This guarantees Hong Kong's "high degree of autonomy" with only limited exceptions such as foreign affairs, defence or in a state of emergency.
Wong said that after U.S. President Donald Trump threatened to impose sanctions on Hong Kong last week, the momentum should be kept to build a "global alliance to stand with Hong Kong".
If China now goes further and imposes national security legislation, this would only risk inflaming the situation.
"I am also anxious the tax structure and changes to the [Hong Kong] tax system may occur", wrote one respondent.
Contributing to simmering anti-government tension, Hong Kong lawmakers are set to resume debate on a bill that would criminalise disrespect of China's national anthem, following scuffles in the legislature in recent weeks.