Sir Elton John has revealed that his son, Zachary, will at some point undergo a paternity test for the sake of his medical records. 

Both Sir Elton and his long-term partner David Furnish contributed sperm to the fertilization process but neither know which is the biological father so will confirm the detail for Zachary's file. The 'Your Song' singer explained:

"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."

He also admitted during his 20/20 interview with Barbara Walters, which aired on April 22, that for the first few month of his son's life breast milk was delivered from the surrogate mother:

"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."

Watch his performance from the opening night of the Tribeca Film Festival below: