British citizen fathers could not pass on their citizenship to their children born abroad before 1st July 2006 if they were not married to the child's mother.
The law changed on 1st July 2006 to allow British citizen fathers to pass on their citizenship to children born to mothers they were not married to. There is now a mechanism where children born to such fathers in the past could register as British citizens, but it has to be applied for and is not automatic.
Read up on the requirements of
Form UKF.
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