U.K. Prime Minister Boris Johnson denounced China's new security law for Hong Kong and said the U.K. would offer residency and a path to citizenship to eligible residents of the semi-autonomous city — potentially numbering in the millions.

What they're saying: Johnson accused China of a "serious breach" of the terms under which the U.K. returned the city in 1997. China pledged to maintain Hong Kong's independent legal system and political freedoms for a period of 50 years.

Details: Foreign Secretary Dominic Raab then described a "bespoke immigration route" for which nearly three million Hong Kongers could be eligible.

• Those with British National Overseas (BNO) status — granted to those living in Hong Kong prior to the 1997 handover — and their dependents will have "right to remain" in the U.K. for five years, during which time they can work or study.
• They will then be allowed to apply for "settled status" and, after an additional year, citizenship.
• Raab added that there will be no quotas. That means the 350,000 holders of BNO passports, plus the 2.6 million eligible to apply for them, could all potentially become U.K. citizens, along with their dependents, per the BBC's Stephen McDonell.

What to watch: "The Chinese Government has been publicly furious with the UK for even threatening just such a move and warned London not to proceed or risk unspecified retribution. Now that this has happened we're waiting for China's response," McDonell writes.

Go deeper:New Hong Kong law sets harsh penalties for broadly defined political crimes