http://www.legislation.gov.uk/id/uksi/1986/948

Loss of British Dependent Territories citizenship3.

Any person who, immediately before 1st July 1997—

is a British Dependent Territories citizen by virtue (wholly or partly) of his having a connection with Hong Kong; and

but for his having a connection with Hong Kong would not be a British Dependent Territories citizen,

shall on that date cease to be such a citizen.