British Citizenship-Unmarried Fathers
In common with many countries, the United Kingdom did not generally allow its nationality to be passed on solely due to an unmarried father. Historically, it was felt that fathers who remained unmarried to the mothers of their children did not usually have a strong link with such children. Over time, social attitudes and laws have changed, and for nationality purposes, British law changed on 1 July 2006.
From this date, as a general rule, an unmarried father is able to pass on British citizenship, provided he is evidenced as the father of the child. Normally this would be through the child's birth certificate, but other evidence, such as a court order or DNA test, may also be acceptable.
The change in the law was not retroactive, in other words, it does not apply to children born before 1 July 2006. However, there are numerous options to get such children British citizenship, as explained below.
 Status of the mother
The situation of the father only needs to be considered if the child is not a British citizen based on the mother's status. In other words, if the child is British based on the mother, they don't even need to look at the father's status.
 Parents marry after the child's birth
The law in its detail is highly complex, but as a general rule if the parents get married after the child is born, then the child becomes a British citizen at that date. There is no age limit.
For an idea of how complex the law can be, it's necessary to read the documents "LEGITIMATION" and "DOMICILE" in Volume 2 of the Home Office Nationality Instructions. If these impact your application (usually they don't, but there are exceptions) then you would need advice from an experienced immigration solicitor.
 Father domiciled in a country, state or territory that abolished the legal concept of illegitimacy
Unusually, a British father may have acquired a domicile (see above) in a country that abolished the status of illegitimacy, as opposed to simply its consequences. If born in New Zealand in 1970 or later, this may be a possibility, for example.
In other jurisdictions you need to get advice in writing from a local family lawyer as to which of the above applies.
 Registration as a British citizen
In early 2000, in anticipation of a change in the law, the Home Office announced it would accept applications for registration as a British citizen under section 3(1) of the British Nationality Act 1981 from children of unmarried British fathers who would have been British if the parents had been married.
Section 3(1) allows the Home Secretary to register any child as British. There are no statutory requirements, only policy requirements that the Home Office set down. Registration as a British citizen is technically "discretionary" but in practice, will be granted if the policy requirements are met. It is not based on the whim of the case-worker.
The most important requirement is that the applicant must be under age 18 at the time of application. There is no flexibility to accept a late application.
For children born outside the United Kingdom the requirements, in addition to being under 18, are:
There is no requirement for the child to be resident in, or hold a permanent settlement visa for, the United Kingdom.
Where child was born in the United Kingdom, there is additional scope to register if the child's father was a permanent resident, or became a British citizen or permanent resident after the child was born.
 Application process
In order to register a child as a British citizen:
With a Certificate of Registration, a child may apply for a British passport. This is not compulsory if there is no plan to move to the United Kingdom, however it is normally advisable.
 Other options
 Immigration visa options
If the child aged 17 or over, he or she may qualify for an Ancestry Visa if there is a United Kingdom born grandparent and the child is a citizen of a Commonwealth country.
It would be unusual for Right of Abode to be an option, but it may be there for someone born before 1983 who:
 Irish citizenship options
Many British citizens have, or are eligible for, citizenship of the Republic of Ireland. The Republic of Ireland abolished the distinctions (for nationality purposes) between married and unmarried fathers in 1987, and unlike the United Kingdom, it did so with retroactive effect. If the parent has a link through birth or descent with the Republic of Ireland or Northern Ireland, then there may be scope to register the child as an Irish citizen.