The Supreme Court decides that a registration condition in the British Nationality Act which is impossible to satisfy for descendants of the female line has to be ignored. Section 5(1) of the British Nationality Act 1948 stipulated the the general rule was that British citizenship was available to a person by descent if his or her father was a citizen of “the United Kingdom and Colonies” at the time of the person’s birth. But, if the person’s father was himself a citizen by descent only, then unless either the person was born in a British-controlled territory or the father was in Crown service at the time of the birth, it was normally a condition under section 5(1)(b) that the person’s birth should be registered at a British consulate within a year. Hence, citizenship by descent could not be transmitted through the female line.

Read more here and the full judgment here.
For details of current and past citizenship legislation in the UK check our our country profile pages.