Mark Langridge, 47, said he donated his sperm as an 'act of kindness' to help two lesbian friends have children.
But 13 years later, his generosity has come back to haunt him after he received a letter from the Child upport Agnecy insisting he has to pay £26 to support the kids he hasn't actually set eyes on in eight years.
Mark, from Essex, who was not named on the birth certificates of the two children, says he cannot afford the payments, which will add up to £8,000 before both children reach adulthood.
He has been in a relationship with his partner Shaun Keeble, 37, for 17 years.
"Me and Shaun were not interested in having children of our own," he said.
"When the idea [of donating sperm] was suggested, we thought it sounded like a lovely thing to do. What could be the harm in helping somebody make their dreams come true? But the last few months have been a complete nightmare.
"I feel as if I am the victim of a state-sponsored blackmail plot ... It was purely an act of kindness on my part and now I am being made to pay."
Mark and Shaun met the women in a gay nightclub in Southend in 1997, and became good friends. When one of the women revealed she was desperate to have children Mark agreed to help, as long as there were no strings attached.
"I did not ask for anything to be documented legally and with hindsight perhaps I should have done," he said.
After the birth of a baby girl in late 1998, Mark was asked to donate again – and a second girl was born in 2000.
When the children were young, Mark remained in touch with the mother, and saw the girls at social events.
He said he was surprised when introduced as their father and later advised the children to ask their mother to explain the facts of their birth.
The two couples eventually drifted apart, and apart from occasional letters from the girls Mark has had no contact with the family since 2004.
In the intervening years the lesbian couple separated, leaving the biological mother with both children. She has since started claiming benefits from the State.
Mark said the biological mother's former partner continues to live near the former family home and sees the girls at weekends – but is not being chased for child support.
He stressed that he made sure the couple had the funds to bring up children before he donated his sperm.
The CSA said that if Mr Langridge had used an official sperm donation centre he would not have to pay child support – but informal arrangements are not covered by the law. Mark argues that the rules should be changed.
The mother of the children said: "It's all being dealt with at the moment and I don't want to comment."