Child born in the UK does not become British Citizen as a right. It depend on the circumstances such as the settlement and nationality of their parents. Different rules apply for the Children of EU Citizens who born in UK or abroad. Child born in UK to British citizen parents and abroad are British Citizen by birth. They do not have to apply for nationality or registration. They can apply for British Passport. If child is not eligible to register, they may be eligible to apply for Naturalisation Application to become British National.
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Naturalisation is a process of getting citizenship of a country through residence. Just like a lot of other countries in the world, UK allows immigration to acquire British Citizenship. UK law allows dual citizenship but the other country, normally country of your Birth, may not allow to have nationality of an other country. Therefore it is very important that you check this before making an application for Naturalisation in UK. India is an example of a country which does not allow dual citizenship.