Hi, thank tou for your question. What is the reason for wanting to do this?
Thank you. Of course it is a difficult situation, but the child has a right to a relationship with her biological father and the mother can only restrict this if there are welfare or child protection concerns. Relinquishing parental responsibility is a major step and her stepfather would need to consider adopting her thereby discharging your son's parental responsibility. You can see further information about the process here: https://www.gov.uk/child-adoption/adopting-a-stepchild
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How did your son acquire PR? Was it through marriage?
How old is the child?
Thank you - the most straightforward method would be for the stepfather to adopt the child.