JULIE ANDREWS, the epitome of English primness, is to reveal in her memoirs that she was born as the result of a brief affair her mother had while married.

The actress famed for her sweet image in Mary Poppins and The Sound of Music will disclose that she was conceived in a passionate liaison her mother enjoyed “with a man by a beautiful lake near Walton-on-Thames”.

At the time of the affair, her mother, Barbara, a professional pianist, was living in outward respectability in Surrey as the wife of a woodwork teacher, Ted Wells.

She later confided to her daughter that “Daddy and I weren’t being very romantic in those days”. So Barbara sought an outlet for her passions elsewhere.

Andrews, 72, has kept the secret of the affair for 58 years. In her memoirs, to be published in Britain by Weidenfeld & Nicolson this month, and in America next month, she writes vividly of the day she learnt the truth.

She was a young girl of four in 1939 when her mother, having divorced Wells after only a few years of marriage, married a Canadian entertainer, Ted Andrews, and the three often performed on stage together. But the young Julie Andrews continued to love Wells as her father.

In the autumn of 1949, when she was already a young singing sensation, having made her solo debut in the successful revue Starlight Roof at the London Hippodrome, her mother asked her to perform at a friend’s house.

“After I had sung, the owner of the house approached me,” she writes. “He was tall and fleshily handsome, and I recognised him as a man who had come round to visit the Meuse once or twice in earlier years.” The Meuse was the home where Andrews lived with her mother and stepfather, whose surname she came to adopt. “That evening the man came and sat on the couch next to me. I remember feeling an electricity between us that I couldn’t explain.”

When driving home, her mother asked if she had liked him. Andrews replied that “he seemed pleasant”. Then her mother asked her why she thought she had been taken there. The teenager pondered for a few seconds before her mother delivered the bombshell: “That man is your father.”

Andrews writes how her brain “slammed into defence mode” as she grappled with the thought that “Daddy” was not her father.

Her mother went on to explain that she had been waiting 14 years to tell her. She said she had no wish to hurt her, but that the affair had been the result of an overwhelming attraction.

Andrews rarely spoke again to her mother about this revelation. “Somehow,” she writes, “I was able to push it to a dark corner of my mind.” But she reveals in her autobiography that Wells remained the man she loved as her “father”, even though he had no biological connection with her. “It did not alter the fact that the man who had raised me was the man I loved. I would always consider him my father,” she writes.

Andrews, who starred on stage in My Fair Lady when she was 21 and won an Oscar at 29 for her performance in Mary Poppins, kept in occasional touch with Wells and has “wonderful memories” of a man who loved language, grammar and poetry. “All his life he committed poems to memory, reasoning he could then return to them any time he wished.”

Interviewed by an American magazine in 2004, she spoke about him without giving any hint he was not her real father. “He was the one who instilled in me any true reality in my life. My father was the one who also took me on nature walks, to swimming baths and down to the seaside. We climbed the local hills; he gave me a love of books. He would buy them for me.”

After her parents’ divorce, she had seen him “occasionally for a two-week visit over the summer holidays, or at Christmas. Or he’d come at a weekend and we’d go on our bikes and ride”. But he remained a strong influence. “What he gave me was always right. Just the memory of him sitting and reading to me was enough to make me love listening to books, and the spoken and written word.”

Wells was remarried to a former hairstylist and they set up home in Hinchley Wood, Surrey. In her memoirs Andrews, who divorced her first husband in 1967 and married the film director Blake Edwards, does not say how Wells died but writes: “I loved him with all my being.” Andrews has had heady moments of her own, defying her squeaky-clean image when she appeared topless in the film SOB, directed by her husband Edwards. In the movie, which blurs the boundary between life and art, she plays a an actress hired for her wholesome image by a desperate director who persuades her to be filmed half-naked.

Andrews’s most recent big film role has been in Shrek 2 and Shrek the Third, as the voice of Queen Lillian, mother of Cam-eron Diaz’s animated character Princess Fiona, the heroine.

There is a moving coda to this tangled history of the actress best known for playing a governess, who spoke so beautifully and almost always kept up appearances. Andrews poignantly reveals that after her mother’s death she asked her aunt Joan for more details about her family. In particular she asked if her “father”, Wells, had known that his wife had been unfaithful and that Julie was not his child.

The aunt told her Wells had known all along she was not his child. “And that simply knocked me sideways,” Andrews reveals.