The Most Revd. Justin Welby says it came as "a complete surprise" when DNA evidence revealed his biological father is the late Sir Anthony Montague Browne.
He had believed his father was Gavin Welby but the Archbishop's mother, Lady Williams of Elvel (pictured below), 86, has admitted going to bed with Sir Anthony in an encounter "fuelled by a large amount of alcohol on both sides".
Justin Welby said in a statement: "This revelation has, of course, been a surprise, but in my life and in our marriage Caroline and I have had far worse.
"I know that I find who I am in Jesus Christ, not in genetics, and my identity in him never changes."
The Archbishop's mother split from Gavin Welby in 1958 (the pair are pictured below at Justin's christening), before marrying Baron Williams of Elvel in 1975.
Lady Williams said: "Although, as has already been made public, Gavin Welby and I had a short and, sadly, dysfunctional marriage, neither of us ever doubted that we were the parents of our son Justin, who was born almost nine months to the day after our marriage in America on April 4 1955.
"I still recall our joy at his arrival.
"So this DNA evidence with which I have now been presented proving that Gavin was not Justin's biological father, so many years after Gavin's death, has come as an almost unbelievable shock."