James Davies, 27, of Stoke Road, Gosport, couldn't understand why the Child Support Agency had taken £220 from his salary.
He has an eight-year-old son, Ryan James Davies, but is no longer required to pay child support.
When he contacted the CSA he was told the money had been taken to pay for another child, aged nine, who he had never heard of. And he was told the only way to stop the payments was to take a DNA test to prove he wasn't the dad.
Mr Davies said: 'I could not understand why my earnings were so short last month. Then I found out I had made a CSA payment.
'At first I thought it was for Ryan but it was for a completely different child.'
Mr Davies first heard of the child when he was sent a letter detailing child-support payments for her in February this year.
When he queried it with the CSA he said it acknowledged its mistake and withdrew the application.
He said: 'I thought that was the end of it and now they've taken money from my employer.
'They've told me the mother of this child is claiming I am the father but I have never heard of her. I don't know this child or this woman. And I am definitely not her father.'
The CSA promised to return any money to Mr Davies if a DNA test proved negative.
sofia.zagzoule@thenews.co.uk