The couple welcomed baby Zachary over Christmas with the help of a surrogate mother and the Rocket Man revealed they both contributed sperm samples to the fertilisation process.

They currently don't know which of them is the child's biological father, but John, 64, has said Zachary will have to undergo testing for his medical records, reported Contactmusic.

"Zach will have to know for his medical records. If it's David, I'd be very proud. It wouldn't matter to me - and vice versa," John told talk show host Barbara Walters.

John also said their surrogate mother sent them breast milk via courier during their first four months after his birth.

"The baby's been brought up in Los Angeles for the first four months so we've had the breast milk 'FedEx-ed' from where she (the surrogate mother) is to Los Angeles every day."