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Minors must be under 18 when applying to be registered as a British Citizen under sections 3(2) or 3(5) of the British Nationality Act 1981.
Is the deadline, in each case, the date of citizenship (so the date on the naturalization certificate) or the date of application?
It would be good to have a source for this information. I hope to reassure a friend who has just made a 3(2) application for her 17 year old daughter.

Date of application.

Section 3(2) begins with

A person born outside the United Kingdom and the qualifying territories shall be entitled, on an application for his registration as a British citizen made while he is a minor...

Very clear. Thank you. Much appreciated.