Some 20,000 children are born every year without a legal right to know their fathers because their parents are unmarried, according to Treoir, the group representing unmarried parents.
Treoir launched an advertising campaign yesterday to stress the importance of allowing children to have contact with both parents. The adverts have the catchline: "It's good for children to know who their parents are."
Margaret Dromey, Treoir chief executive, said it was difficult to say how many children did not have any contact with their fathers but about one-quarter of children born outside marriage did not have their father's name on their birth certificate. While other children may have their father's names on their birth certificate, they may not have any contact with him.
Ms Dromey said Treoir "passionately" believed contact with both parents was good for children except in circumstances of abuse or violence. "We would say that, at a minimum, parents should ensure that both their names are on the child's birth cert."
She said it was not about a mother's rights versus a father's rights. It has always been about the needs of a child in relation to both parents.
She said all parents wanted the best for their children. "But clearly if parents are in conflict it's extremely difficult to organise contact or to enter into co-parenting arrangements."
Ms Dromey said many unmarried fathers lost contact or never developed a relationship with their child.
"They may do this to avoid conflict with the child's mother, particularly if she has a new partner, or because she is upset because he is in a new relationship."
Pat Bennett, chief executive of the Family Support Agency which funded the campaign, said the campaign was about the right of the child to know both parents and to be able to contact them.
He said parents must seriously consider whether it was in the best interest of their children to exclude the second parent from their lives.
"We have all seen the devastation and the impact on children's lives, and more so when they come into their adult lives, in not having contact with a parent.
"By and large, we know it's very important for a child to have access to both parents. It's very important for both parents to have access to the child, and that's the other side of the story."
He pointed to the UN Convention on the Rights of the Child, ratified by the Government in l992, which states that children have a right to have contact with both parents. It also states that both parents have common responsibilities with their children.
Last year Treoir received more than 400 calls from fathers who wanted to be involved in their children's lives but were wondering if it would be better to opt out because of acrimony with the mother.
