Children adopted by a British citizen, either in the UK or in a country signatory to the Hague convention, acquire British citizenship by adoption. These children do not have to be registered as British citizens, but may apply for a confirmation of their British citizenship for the purpose of applying for a British passport.
Unlike adoptions under the Hague convention, other inter-country adoptions do not give rise to automatic acquisition of British citizenship. Children adopted by a British citizen abroad in one of the designated countries whose adoption procedures are recognised by the UK family courts, may be registered as British citizens while they are under the age of 18. Others have to obtain a parental order or go through a recognised adoption procedure, or rely on discretion of the Secretary of State to register them as British citizens where they would be severely prejudiced if such discretion is not exercised.