Andy Bathie works as a firefighter

Andy Bathie, 37, from Enfield, north London, claims he was assured by the couple he would have no personal or financial involvement for the children.

He donated his sperm as a friend rather than go through a fertility clinic.

The CSA said only anonymous donors at licensed centres are exempt from being treated as the legal father of a child born as a result of their donation.

Mr Bathie, a firefighter, said he cannot afford to have children with his own wife due to the financial implications.

Shock, anger and despair

The lesbian couple, who approached the couple five years ago after they married in a civil ceremony, have a boy and a girl.

Mr Bathie said he reacted with "shock, anger and despair", when he was contacted by the CSA in November.

He said: "I don't have any particular ill will. It's the fact that I still even now don't see why I should have to pay for another couple's children."

A spokesman for Human Fertilisation and Embryology Authority (HFEA) said: "The law says that men donating sperm through licensed fertility clinics are not the legal father of any child born through that donation.

"Men giving out their sperm in any other way - such as via internet arrangements - are legally the father of any children born with all the responsibilities that carries."

Fertility reforms

A spokeswoman for CSA said: "Unless the child is legally adopted, both biological parents are financially responsible for their child - the Child Support Agency legislation is not gender or partnership based."

Ministers have drawn up fertility reforms giving equal parenting rights to same-sex couples who "marry" in a civil partnership.

This means they will be recognised as the legal parents of children conceived through sperm donation.

The change comes too late for Mr Bathie, although he is now pushing for an amendment to make the laws retrospective.