Lawyers said that the decision was a breakthrough in the treatment by the courts of same-sex relationships.

Lord Justice Thorpe, the deputy head of the Family Division, reversed a ruling made at Telford County Court last November which denied a joint residence order to the first partner.

By winning joint residence, the 46-year-old woman now has shared parental responsibility for the girls, aged 6 and 3.

This is the only method under present law that a same-sex partner who is not a biological parent can claim such responsibility.

Lord Justice Thorpe said that firm measures needed to be taken to safeguard a vital side of the children’s family life.

It was the first case of its kind to reach the Court of Appeal, which imposed reporting restrictions barring identification of any of the parties involved.

The woman, who has a 16-year-old son of her own, had lived with the mother of the girls, who were conceived by donor insemination, since 1995. Their relationship ended in May 2003 and the mother, 31, now has a new partner.

During the county court proceedings it was said that the mother had tried to marginalise her former lover and their attitude towards one another had become hostile.

Although the county court granted sole residence to the mother, the judge did allow the former partner contact so that she could have “a significant role in the lives of the children”. But the mother then announced that she would be moving to Cornwall with her new partner, making contact difficult. The ruling does not mean that the former couple will have to return to living together in the same house. It is merely a device to ensure that the non-biological parent has responsibility for and contact with the children.

In practice, the woman who won the ruling may find it easier to move nearer to Cornwall to achieve that contact.

Lord Justice Thorpe said that it was a feature of the appeal that parties in same-sex relationships can achieve parental responsibility only if they succeed in an application for a joint residency order.

He said that there was now a trend towards “relaxation of the boundaries” of the rights of same-sex partners but, at the moment, the route to gaining parental responsibility was only through joint residency.

Lord Justice Thorpe said that a court welfare officer had reported to the county court that the appellant wanted “proper legal involvement in the girls’ lives”, adding: “She wanted to love, support and help them.”

The mother believed that her former partner should be seen as an extended family member, not a parent, and did not accept that the former lover could care for the children properly.

The court welfare officer said that excluding the former lover would not help the children, who needed to understand the history of their earlier lives.

“A shared residency order would help the girls to have a clear picture of where they fit in when they grow older,” the officer said. She said that the mother’s move to Cornwall was an attempt to frustrate the court’s contact arrangement.

But the county court judge ruled that joint residency would lead to endless disputes between the former partners, which could require the court’s intervention and was not in the best interests of the children.

Yesterday, overturning that decision, Lord Justice Thorpe said that the children would benefit from allowing shared parental responsibility between the former partners.

He said: “What has been said about the importance of fathers is of equal application in same-sex parents.”

The appeal judge said that there had been a “judicial recognition” of family diversity as the family had evolved in modern society. “I am in no doubt at all that the children require firm measures to safeguard them from diminution or loss of a vital side of family life.”

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