Andy Bathie, 37, said the women, who approached him five years ago after other male friends declined to become donors, assured him he would have no personal or financial involvement in the children's upbringing. But he said the Child Support Agency contacted him last November and made him take a £400 paternity test, then demanded support payments because the couple had split up.

Legally, only men who donate sperm through licensed fertility clinics do not become the father of any children conceived using their donation.

Proposed legislation, at committee stage in the House of Lords before passing to the Commons, would give equal parenting rights, including financial responsibilities, to both members of same sex couples, but the change will come too late for Bathie, who is lobbying for the laws to be made retrospective and for him not to be seen as the legal parent of the children, now aged two and four.

"These women wanted to be parents and take on the responsibilities that brings. I would never have agreed to this unless they had been a committed family. And now I can't afford to have children with my own wife - it's crippling me financially," he told the Evening Standard.

At the time of his donation, Bathie was in a relationship with a woman who was not planning to have children, but he has since married someone else.

"I did look into the legal side and understood that as a couple they would be the parents, not me. I was never daddy. They wanted children as a couple, which means they should take responsibility. The CSA admit that mine is an unusual case - this is double standards."

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

Natalie Gamble, a fertility law expert at Lester Aldridge, who has advised Bathie, said the case was the first of its kind she had come across. "Currently a non-birth mother in this situation is not automatically recognised as the parent in law, so she is not financially responsible. If the law being proposed was to apply in Andy's case, his responsibility for the child would be passed to the non-birth mother."

The Human Fertilisation and Embryology Authority warned that "DIY" donors using methods such as unlicensed websites or home insemination were financially liable for their offspring.

Phil Willis MP, chairman of the innovation, universities and skills select committee, which deals with human fertilisation and research, said: "The CSA has to look very carefully at the issue. I suspect he won't get his money back, as there would be a flood of similar applications."