The Archbishop of Canterbury Justin Welby has discovered that his biological father is not Gavin Welby but the late Sir Anthony Montague Browne, the private secretary of Sir Winston Churchill.

"This comes as a complete surprise," he said. But he also said it changed nothing.

"I find who I am in Jesus Christ, not in genetics, and my identity in him never changes," he added. "This is a story of redemption and hope from a place of tumultuous difficulty and near despair in several lives. It is a testimony to the grace and power of Christ to liberate and redeem us, grace and power which is offered to every human being."

His mother said the revelation was an "almost unbelievable shock" but that in spite of many issues around alcoholism, her family now was truly blessed. "None of this would have been possible without our firm Christian faith and a determination never to relinquish hope."

The news was revealed in The Daily Telegraph after a DNA test that he took to quell rumours about his parternity. He and his mother, Jane Williams, who was Sir Winston's personal secretary at the time, had never doubted that Gavin Welby, a whiskey salesman and alcoholic, was his father.

Sir Anthony's widow however had always suspected the truth because of the strong resemblance between Archbishop Welby and her late husband.

In a deeply moving statement, Archbishop Welby said both his mother and father were alcoholics: "My mother has been in recovery since 1968, and has not touched alcohol for over 48 years. I am enormously proud of her."

His father died as a result of alcohol abuse and smoking in 1977 when Justin was 21.

"As a result of my parents' addictions my early life was messy, although I had the blessing and gift of a wonderful education, and was cared for deeply by my grandmother, my mother once she was in recovery, and my father (Gavin Welby) as far as he was able," said Archbishop Welby.

"I have had a life of great blessing and wonderful support, especially from Caroline and our children, as well as a great many wonderful friends and family.

"My own experience is typical of many people. To find that one's father is other than imagined is not unusual. To be the child of families with great difficulties in relationships, with substance abuse or other matters, is far too normal.

"By the grace of God, found in Christian faith, through the NHS, through Alcoholics Anonymous and through her own very remarkable determination and effort, my mother has lived free of alcohol, has a very happy marriage, and has contributed greatly to society as a probation officer, member of the National Parole Board, Prison Visitor and with involvement in penal reform."

He continued: "She has also played a wonderful part in my life and in the lives of my children and now grandchildren, as has my stepfather whose support and encouragement has been generous, unstinting and unfailing.

"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. Even more importantly my role as Archbishop makes me constantly aware of the real and genuine pain and suffering of many around the world, which should be the main focus of our prayers."

In a personal statement, his mother, now Lady Williams of Elvel 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."

She said her son deserved an explanation and she had been as open as she could be, given the passage of more than sixty years with all the lapses in memory that age entails. "But I accept that what would in normal circumstances be of interest only to those immediately concerned must now, because of my son's position in public life, be given wider circulation. So I write this in order that the matter be put to rest."

She said Gavin Welby had been a very strong, possessive character. At the end of March 1955 he was bullying her to leave her job as personal secretary to the Prime Minister and run away with him and marry him in the United States where his divorce was being finalised. At the age of 25, the pressure became too great and in the end she found herself unable to resist.

"One feature of this pressure is that I was already drinking heavily at times. Although I could then ensure that this did not affect my work, it was later to develop into serious alcoholism during the 1960s which only came to an end when I entered rehab in 1968. I have not drunk alcohol since.

"Although my recollection of events is patchy, I now recognize that during the days leading up to my very sudden marriage, and fuelled by a large amount of alcohol on both sides, I went to bed with Anthony Montague Browne. It appears that the precautions taken at the time didn't work and my wonderful son was conceived as a result of this liaison."

After leaving her job and getting married, she did not see Sir Anthony again for a long time. "After Gavin and I broke up in 1958 Anthony and I met occasionally but although he may have asked how Justin was, there was nothing that gave me any hint that he might have thought he was Justin's father."

She continued: "My beloved husband Charles Williams and I have enjoyed a loving and stable marriage from 1975 to the present day. With that stability and love I have been able to blossom as never before. I have served on the Parole Board, as a magistrate, as a member of a Board of Visitors of a prison, as chair of a Howard League committee and as a deputy lieutenant for Greater London. Even at the age of 86 I lead an active life both in support of my husband and in my own right.

"Furthermore, I have watched Justin, from an almost impossible childhood (Gavin was alcoholic as well), grow into what he is today, marry his beautiful wife Caroline in 1979 and see his children and now grandchildren grow up around them. As a family we are truly blessed. But none of this would have been possible without our firm Christian faith and a determination never to relinquish hope. God has given us so much and my gratitude knows no bounds."

One response came from a friend of the Archbishop's, the head of the UK Coptic church, Bishop Angaelos. He said: "I found the archbishop's [statement] to be very moving indeed, and indicative of the peace, love, forgiveness and resolve that we are not only all called to, but all endowed with if we but allow God's healing, reconciling and comforting presence in our lives. This is of course also indicative of the personable and 'real' character we have all come to know to be Justin Welby, who, I am confident, will have this experience further enrich his ministry of compassion."