The Archbishop of Canterbury has said a DNA test result revealing the identity of his biological father has come as a "complete surprise".
The Most Reverend Justin Welby, 60, has discovered he is the son of Sir Winston Churchill's last private secretary, the late Sir Anthony Montague Browne.
He had believed his father was whisky salesman Gavin Welby, who died in 1977.
His mother, Lady Williams of Elvel, has confirmed she had a "liaison" with Sir Anthony just before she wed in 1955.
She said the news had come as an "almost unbelievable shock", and although her memory of the time was patchy, she recalled sleeping with former colleague Sir Anthony after "a large amount of alcohol on both sides".
Archbishop Welby, the most senior cleric in the worldwide Anglican communion, said he discovered the real identity of his biological father in recent weeks.
The Daily Telegraph reports that he decided to take a DNA test after it approached him with research it had carried out into his family's background.
It reports mouth swabs were compared with hair samples from a hairbrush kept by Sir Anthony's widow and showed a 99.9779% probability they were father and son.
Former Telegraph editor Charles Moore, who broke the story, said the archbishop was "very, very surprised " when the DNA tests were revealed to him.
"The dates looked impossible because he was born almost exactly nine months after his mother married Gavin Welby, and so it was natural to think that he was a honeymoon baby."
He told Radio 4's Today programme that it appeared that Lady Williams went to bed with Sir Anthony in the days before she eloped to the US with Gavin Welby.
By BBC religious affairs correspondent Caroline Wyatt
Justin Welby has lived a life worthy of a protagonist in a Charles Dickens novel, with twists and turns at every new chapter. He is perhaps one of the most unusual Archbishops of Canterbury that the Church of England has ever had.
On the surface, his confidence and charm seem typical of the sheen that an education at Eton bestows. He went on to become an oil executive, and travel the world. But for all that apparent privilege, his own childhood, and the younger lives of both the man he assumed to be his father, Gavin Welby, and his mother Jane, were deeply affected by his parents' addiction to alcohol.
And yet, as he and his mother make clear in their public statements, this is not a story of misery but one of redemption, with Justin Welby and his mother both crediting their families and their faith for helping her to overcome alcohol, and for helping him to show that children can overcome a difficult start in life.
In a statement issued through Lambeth Palace, the Archbishop of Canterbury refers to his mother and Gavin Welby's addiction to alcohol while he was growing up.
"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."
He added: "I know that I find who I am in Jesus Christ, not in genetics, and my identity in him never changes...
"Although there are elements of sadness, and even tragedy in my father's (Gavin Welby's) case, this is a story of redemption and hope from a place of tumultuous difficulty and near despair in several lives."
Lady Williams, whose marriage to Gavin Welby broke up in 1958, had worked as secretary to Sir Winston during his final spell as prime minister.
In a statement published online she said: "Although my recollection of events is patchy, I now recognise 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."
In an interview with the Telegraph, the archbishop said the news had not affected his relationship with his mother, who always looked reality "straight in the face".
"I wasn't in any way upset... and I remain not upset," he added.
The Bishop of Norwich, Graham James - who has been talking to the archbishop over the past week - told BBC Radio 5 live Mr Welby decided to take the DNA test to disprove the claim.
"It proved quite the opposite and he discovered this for certain just before Easter and then contacted his mother... his first thoughts were for her rather than anyone else," he said.
"Although she knew the history of her relationship with Sir Anthony at the time, she'd always believed, as had Gavin Welby, that they were Justin's parents."
The Roman Catholic Archbishop of Westminster, Cardinal Vincent Nichols, said he was "full of admiration for the dignified manner in which [Mr Welby and Lady Williams] have handled this startling news".
"Upheaval in family life is neither uncommon nor easy to embrace. Every family knows this. But to do so with such steadiness and honesty in the full glare of publicity is remarkable and yet fully characteristic of them both.
"This is a great tribute to their Christian faith."
Who was Sir Anthony Montague Brown?
- Born in May 1923, he was the son of a British army colonel, according to his Daily Telegraph obituary
- He went to Stowe school in Buckinghamshire before attending Magdalen College, Oxford
- Sir Anthony joined the RAF and served in the Middle East and Asia during World War Two
- He started working for the Foreign Office after the war and became private secretary to Sir Winston while he was in Downing Street in 1952
- His role continued for 10 years after Sir Winston resigned as prime minister in 1955, until the former prime minister's death
- Sir Anthony died in 2013, aged 89
Lady Williams, who remarried in 1975, says she has not drunk alcohol since 1968.
She added that she had seen Sir Anthony occasionally after her relationship with Gavin Welby ended.
"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 said.
Educated at Eton and Trinity College, Cambridge, and brought up an only child, Mr Welby spent 11 years as an oil executive, before giving up a six-figure salary in 1987 to train as an Anglican priest.
The decision followed a personal tragedy in 1983 with the death of his seven-month-old daughter, Johanna, in a car crash.
In 2002, he became canon at Coventry Cathedral, before serving as Dean of Liverpool Cathedral and Bishop of Durham. He succeeded Rowan Williams as Archbishop of Canterbury in February 2013.