Sir Elton John and David Furnish will never reveal their baby's biological father.

The couple, whose son Zachary was born to a surrogate mother on Christmas Day (25.12.10), have made a pact to keep the process of the tot's conception a secret - so he grows up seeing his two dads as equals.

A friend of David's told The Sun newspaper: "He and Elton made a pact that they are equal parents - no one will ever know who the biological father is.

"They will deal with whether Zach is told, if and when the time arises in the years ahead."

However, the 63-year-old singer has admitted that he wanted the child to be his - but the couple may have chosen David, 48, as the donor because he is younger.

A friend said: "Elton was clear he wanted his bloodline to continue but also took on board advice that a younger man may be a more appropriate donor. It is a very complicated arrangement - but one forged by love."

According to reports, Elton is listed on Zachary's birth certificate as his father, while David is mentioned as mother.

Meanwhile, Elton and David are using their new fatherhood status to renew their bid to help two HIV-positive brothers, Lev and Artyom, they tried to adopt in the Ukraine.

David said: "Although Ukranian law does not permit Elton and I to adopt Lev and Artyom, we are determined to give them the best chance of living a happy and healthy life with a loving Ukranian family.

"The arrival of our son has done nothing but strengthen our resolve to help these two wonderful children.

"We are, however, constrained by Ukranian policy and legal procedure, and this is why for the past 12 months we have retained a leading Ukranian law firm to assist us in this process."