Men who donate sperm can apply to seek a role in the lives of their biological children, the High Court has ruled.
The decision could have implications for families using donated sperm and donors, who have no legal role as parents of their biological child.
Mr Justice Baker ruled that two men, whose sperm was used by lesbian couples they were friends with, could apply through the courts for contact.
The judgement does not mean that any future application would be successful.
The cases, heard at the High Court's Family Division, centred around two men in a civil partnership and two lesbian couples they were friends with.
There was contact with the couples after the birth of the children involved. However, there were disagreements about the level of the men's involvement and they applied to the courts.
Mr Justice Baker said that lesbian couples and their children had exactly the same legal status as any other parent and child, however "it is still open to the court, after considering all relevant factors, to grant leave to other persons to apply for (contact) orders".
He ruled that, in this case, it was "appropriate" for the biological fathers to apply for a contact order, which would set out when they could see the child such as on certain weekends or during school holidays.
Kevin Skinner, who is from the firm Goodman Ray and represented one of the lesbian couples in the case, said: "Although the judge's decision makes clear that the family unit should be protected, the possibility of donors being able to apply for courts orders will be a scary prospect for many parents, both gay and straight.
"What is crucial is that anyone planning on having a child through the use of fertility treatment should make sure that proper plans are in place before the process begins."
However, it is thought the cases may be solved through mediation rather than returning to the courts.
The fertility regulator, the Human Fertilisation and Embryology Authority, said: "This raises the question of whether, following donor conception treatment, a known donor can seek access to a child that he is biologically related to, even if he is not the legal father.
"The case has not yet been heard so we do not know whether access will be granted.
"If contact is granted this may raise concerns for families who have had donor conception treatment using a donor known to them - whether through a private arrangement or through a licensed clinic."