The California-based clinic used by Elton and David proclaimed the happy news of their involvement in the birth of the singer's son Zachary, on its website.

The Centre For Surrogate Parenting in Encino which helped the couple realise their wish to become parents said in a statement: "CSP is proud to have helped David Furnish and Elton John become the proud parents of a baby boy on Christmas Day."

Joanne Bubrick, programme administrator, said: "I can confirm that Elton John and David Furnish worked with the Centre for Surrogate Parenting and it was a lovely, beautiful surrogacy journey for all parties involved." Elton and David fulfilled their dream of having a child when their son Zachary Jackson Levon Furnish-John was born in Los Angeles on Christmas Day, weighing 7lb 15oz.

He was born to an unidentified surrogate mother who, it emerged last night, was not his biological mother but had carried the baby after being implanted with a donor egg from another mystery woman. The complicated arrangement carried out by the Encino, California-based Center For Surrogate Parenting Inc - a favourite with Hollywood stars - means that little Zachary effectively has two fathers and two mothers.

The centre previously arranged for Frasier actor Kelsey Grammer and his ex-wife Camille to have two children by the same surrogate mother - their daughter Mason Olivia, in 2001 and son, Jude Gordon, in 2004. And actress Angela Bassett and husband Courtney Vance became parents of twins, a boy and a girl, who were born through a surrogate in January 2006. Days Of Our Lives soap star Deidre Hall and former newsreader Joan Lunden also used the centre's services to become parents. Elton and David meanwhile successfully kept Zachary a secret from the world until three days after his birth.

They had previously been turned down for adoption because of Elton’s age. And the former drug addict recently said it would not be ‘fair’ on a child because of his advancing years and the amount of travelling he did. He is due to begin a gruelling worldwide tour next month. Thanks to California’s surrogacy-friendly laws, Elton and David are likely to have become Zachary’s lawful parents straight away.

But the birth raises a number of questions. Nothing is known about the surrogate mother, her background, religion or marital status, and whether she has any rights over the child. The same applies to his biological mother, the woman who donated an egg for the process known as ‘gestational surrogacy’. Another unknown is which man is the baby’s true father – if either of them.

No matter how puzzling his parentage, Zachary is likely to have A-list stars as his godparents. Elton is godfather to David and Victoria Beckham’s son Brooklyn, 11, and last night Mrs Beckham said: "David and I are absolutely thrilled for Elton and David. They have so much love to give and will be wonderful parents."

Elizabeth Hurley, 45, is also lined up as a potential godmother. The actress used the singer’s country estate as a bolt-hole after the arrival of her son Damian, now eight, and made Sir Elton the boy’s godfather as a thank-you. A spokesman for Elton said the proud fathers were still in LA. In a statement, the couple said: "We are overwhelmed with happiness and joy at this very special moment. Zachary is healthy and doing really well, and we are very proud and happy parents."

The identity of the surrogate mother is a closely guarded secret. But unlike elsewhere in the world, it is not illegal in California for a multi-millionaire such as Elton to pay a significant sum of money to buy a baby. If she was paid, a surrogate mother could in theory have become very rich after successfully delivering Zachary. It is likely she would have signed a strict confidentiality agreement never to speak about the deal.

Nor has it been announced which, if either, of the men is the biological father. However last night friends of the couple speculated that David was the true father. NHS experts recommend sperm donors should be under 40, because evidence suggest genetic abnormalities are more common in older fathers. If they had followed such advice, it is more likely that David’s sperm was used in the conception.

Elton and David have opted for traditional names for their son. Zachary is of Hebrew origin, and means ‘The Lord recalled’, while Jackson is of Old English origin and simply means ‘Son of Jack’. The name Levon is the title of a track on Sir Elton’s 1971 album Madman Across The Water, which contains the line: "He was born a pauper to a pawn on a Christmas day." Zachary is the first child for both men. The couple made their surprise announcement of his arrival to American magazine Us, adding that out of respect for the surrogate mother’s privacy they would not disclose any further information.

When his long-term keyboardist, Guy Babylon, died of a heart attack aged 52 in September last year, Elton said: "What better opportunity to replace someone I lost than to replace him with someone I can give a future to?" But he and Mr Furnish then made a failed attempt to adopt a 14-month-old HIV-positive boy called Lev from an orphanage in Ukraine. They were rejected because of Elton’s age and the fact that the country did not recognise gay partnerships. Later, the singer suggested that having a child could be inappropriate. He said: "David always wanted to adopt a child and I always said no because I am 62 and I think because of the travelling I do and the life I have, maybe it wouldn’t be fair for the child."

The baby’s arrival led to an outpouring of gushing messages from a host of his fathers’ celebrity friends. Model and actress Elizabeth Hurley was the first to congratulate them on Twitter, writing: "Massive congratulations to David and Elton on having their beautiful son. Can’t wait for my first cuddle."

Sir Michael Caine offered his wishes, saying: "Great news about Elton and David adopting a child, let’s hope more people are granted adoption." Apprentice star Lord Alan Sugar expressed disbelief at the news, tweeting: "Am I hearing things right on Sky news Elton John becomes a surrogate father." He added about an hour later: "Oh well congratulations to him."

And TV Presenter Piers Morgan, who recently interviewed Sir Elton for his ITV series Life Stories, was also caught out by the unexpected news, writing: "Feel such a fool – Sir Elton never even hinted he was pregnant during our interview." Former Spice Girl Emma Bunton wrote: ‘Congratulations to Elton and David’, while singer Boy George wrote on his Facebook site: "Wow, surprise news about the baby, congratulations!"

Amanda Holden tweeted: "So thrilled for Elton John and David Furnish! How wonderful life is – when you become parents!"

Mr Furnish had hinted at the looming arrival on Christmas Day when he wrote on his Facebook page that he was "on course to have the best Christmas ever!" Hours later he broke the news of Zachary’s birth and revealed that he had never believed his dream would come true. In a message to a friend, the Canadian said: "I’m overwhelmed… I never, ever, thought I would be a dad."

The new fathers have already spent £35,000 designing and decorating a nursery for their son. The pair have enlisted the help of their friend shoe designer Patrick Cox to help them put the finishing touches to the ‘palatial’ room at the their Windsor mansion, Woodside. They are said to have splashed out thousands of pounds furnishing the room with ‘one-off, tailored furniture items’.

A source said: "Elton and David began planning the nursery months ago. They wanted it to be ready for when the baby came home. It will be very lavish when it is finished. They like to make big statements." It is a clear indication that the couple plan to bring up their child in Britain, even though they will have to wait a few weeks to bring him home.

The friend added: "England is where their heart is. There is no way they would move to America full time."

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