The fundamental principle of the Legitimacy Act 1926 is exposed in article 1(2): "Nothing in this Act shall operate to legitimate a person whose father or mother was married to a third person when the illegitimate person was born." The Act allowed children to be legitimised by the subsequent marriage of their parents, provided that neither parent had been married to a third party at the time of the birth. In those circumstances, the legitimised birth was re-entered in the birth indexes for that year (sometimes many years after the original birth). The original entry would be annotated to refer to the new entry.^[1]

The act was modified by the Legitimacy Act 1959, which extended it to children whose parent(s) had been married to somebody else when they were born.^[1]