United Kingdom - Passport & Nationality - British Citizenship by Descent (British Parent post 1983)
A Passport by Descent from a British parent is available under this category where the parent - at the time of the birth - was British other than by descent. This is per the UK Nationality Act of 1981 Section 2-1.
A British Citizen Other than by Descent is someone who achieved this status under the following four circumstances:
1) Mother with a UK Passport married to a man holding a UK Passport who qualifies for points 2-4 below;
2) A person born in the UK before 01/01/1983;
3) A person naturalised or UK Registered before 01/01/1983; OR
4) A person born in a UK Colony AND has a UK Passport, but has no parent or grandparent born in the UK.
In addition, if the birth took place BEFORE 01.07.2006, and the birth was not legitimate - that is, the parents were not legally married at the time of the birth, then a separate application for a British Passport is still required.
Click here for more information on how you could claim British Nationality.