Arthur Miller hid the existence of a son with Down’s syndrome for decades, but then quietly included him in his will weeks before his death.

The playwright committed the baby boy to an institution when he was a week old and cut him out of his life, failing even to mention him in his memoir, Timebends.

The child, named Daniel, now 40, is the son of Miller’s third wife, the late Inge Morath, a Magnum photographer whom he met on the set of The Misfits, when she was taking pictures of Marilyn Monroe, his second wife.

The secret son is the younger brother of Rebecca Miller, the actress and Personal Velocity director, who is married to the actor Daniel Day-Lewis. Quoting friends and family, Vanity Fair magazine suggests that an “appalled” Day-Lewis, who played a disabled person in his breakthrough role in My Left Foot, may have pressed Miller to renew contact with his son in the late 1990s.

Six weeks before his death in 2005 at the age of 89, Miller wrote a will and signed confidential trusts leaving Daniel a share of his estate equal to his other three children – Rebecca, and Jane and Robert, his son and daughter from his first marriage to Mary Slattery.

Rebecca Miller was at a loss to explain her father’s treatment of Daniel. “The only person who can truly answer your questions is my father, and he is dead,” she told Vanity Fair.

The playwright’s behaviour appears at odds with the principled stances he took throughout his life, from his refusal to “name names” of communist sympathisers to his opposition to the Vietnam War and his work for oppressed writers.

Suzanna Andrews, the author of the Vanity Fair article, notes that the boy was born after Miller completed his best work, plays such as Death of a Salesman and The Crucible. “A writer, used to being in control of narratives, Miller excised a central character who did not fit the plot of his life as he wanted it to be,” she writes. “Whether he was motivated by shame, selfishness, or fear – or, more likely, all three – Miller’s failure to tackle the truth created a hole in the heart of his story. One wonders if, in his relationship with Daniel, Miller was sitting on his greatest unwritten play.”

The existence of a hidden child was first mentioned in a 2003 biography of Miller by the American theatre critic Martin Gottfried, who gave the apparently erroneous birth date of 1962.

Mr Gottfried said yesterday that he thought Miller’s reputation as a playwright was secure. “All that theatre people care about are the works themselves,” he said. “In England, where Miller is considered an equal of Chekhov, all they care about is that Miller is a great, great playwright.”

“If he wrote Death of a Salesmanand that’s all, that would be enough,” he said. “He could be the worst son-of-a-bitch who ever lived, but he still wrote those plays.”

Vanity Fair said that Daniel was born in November 1966 and was committed to an institution within days in an effort to protect Rebecca from living with a brother the playwright described as a “mongoloid”.

“Inge wanted to keep the baby, but Arthur was not going to let her keep him,” a friend told the magazine.

Daniel was placed in a home for infants in New York, before being sent at age 4 to the 1,600-acre Southbury Training School for the mentally retarded in Connecticut, which housed almost 2,300 people in rooms containing 30 to 40 beds.

“Inge told me that she went to see him almost every Sunday, and that [Arthur] never wanted to see him,” the writer Francine du Plessix Gray said.

Conditions were said to be poor. Jean Bowen, a disability rights activist who visited Daniel at the school, said that his sole possession was a tiny transistor radio with earplugs.

Daniel was released at age 17 and went to live in a group home with five housemates. He then moved on to a “supported-living programme” that enabled him to stay in a flat with a roommate, with someone checking in on him once a day. He held a job, first at a local gym and later at a supermarket. A natural athlete, he competed in the Special Olympics in skiing, cycling, track and bowls.

In 1995 Daniel met his father in public for the first time when he attended Miller’s speech in support of Richard Lapointe, a disabled man who had been convicted of murdering his wife’s grandmother. Daniel, who was at the event with a disability rights group, ran up and embraced the playwright, who gave him a big hug. “Danny was thrilled,” Ms Bowen recalled.

In the late 1990s Miller went to a two-hour meeting about Daniel’s care. “He was absolutely amazed at Danny being able to live out on his own,” Rich Godbout, a social worker, told the magazine.

Daniel is now living with an elderly couple who have long taken care of him, in a sprawling addition to their home that was built specially for him.

Rebecca Miller insisted that “Danny is very much part of our family” and said he “leads a very active, happy life, surrounded by people who love him”.

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