The British Citizenship (Designated Service) Order 2006
Section 2 of the British Nationality Act 1981 provides that a person born outside the United Kingdom and the qualifying territories after commencement of that Act will be a British citizen in certain circumstances, including where at the time of the birth his father or mother is a British citizen and is serving outside the United Kingdom and the qualifying territories in service designated by the Secretary of State, recruitment for that service having taken place in the United Kingdom or a qualifying territory. The Secretary of State may only designate for this purpose service which he considers to be closely associated with the activities outside the United Kingdom and the qualifying territories of Her Majesty’s government in the United Kingdom or in a qualifying territory.