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.
- If the child is born in the United Kingdom, Channel Islands, Isle of Man, or (after 21 May 2002), a British Overseas Territory, then then child will still be a British citizen if the mother is a British citizen or a permanent resident of the United Kingdom when the child is born.
- If the child is born elsewhere, and the mother is a British citizen otherwise than by descent.
 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:
- the child would have been British if the parents had been married. In particular, if the father is a British citizen by descent then they will only register the child if the child would have met the requirements for registration normally; and
- there is suitable evidence of paternity.
- if age 10 or over, the child is of "good character"; and
- there is no special reason to refuse registration. Normally the consent of both parents would be required, however they do not give a parent a veto over the application. A "good reason" might be the fact that the child would lose the nationality of the country of residence.
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.
Home Office information on British citizenship for children of unmarried fathers
 Application process
In order to register a child as a British citizen:
- Form MN1 needs to be completed for each child with fee and supporting evidence.
- From April 2009, the fee will be GBP460. Additional children applying at the same time require a fee of GBP50 each (up to the fee change, a single fee of GBP400 covers multiple children).
- If child is outside the United Kingdom and the British territories, application needs to be submitted to a British diplomatic mission. They will forward the application to the Home Office for decision.
- A consular fee will be levied, in addition to the above fees (normally payable in local currency).
- Processing time for stand-alone child registrations is normally quite quick, no more than a few months in most cases.
- The Home Office will send the child's Certificate of Registration as a British citizen to the diplomatic mission, who will contact you.
- If child has turned 18 during processing, child will need to attend a citizenship ceremony.
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:
- has a United Kingdom born mother; or
- was adopted by British parents; and
- has continuously been a citizen of a Commonwealth country since 1 January 1983.
 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.