The child's mother is believed to be Swedish TV producer Anna Elisabet Eberstein. She is said to have first registered the birth in September 2012, leaving the details of the father blank.
A new entry was re-registered in December, listing the daddy as Hugh John Mungo Grant.
The baby - whose name is not known - arrived three months before his second child with Chinese actress Tinglan Hong.
Baby Felix Chang was born at St Mary's hospital in Paddington on 29 December 2012, becoming a little brother for Hugh and Tinglang's daughter Tabitha. She made her entrance in September 2011.
Hugh announced the birth of Felix on his Twitter account, posting: "In answer to some journos. Am thrilled my daughter now has a brother. Adore them both to an uncool degree. They have a fab mum."
He added: "And to be crystal clear. I am the Daddy."
He is yet to comment on these new stories claiming he is a dad of three.
Six months after Tabitha's birth, Hugh spoke to the Guardian newspaper about becoming a father for the first time.
He told the newspaper that he was finding it exciting getting to know his baby, though he was told not to expect much at first.
"Lots of people warned me about that; they said never let anyone know, but the baby period is not that exciting.
"But I am excited, actually. I thought, well, I'll bluff through – but very little bluffing has been required."
Hugh chose his daughter's English name, Tabitha, whilst Tinglan chose a Chinese name, Xiao Xi, which means "happy surprise."
He added: "I like my daughter very much. Has she changed my life? I'm not sure. Not yet. Not massively, no. But I'm absolutely thrilled to have had her, I really am. And I feel a better person."