Non religious people are better at making reasonable, compassionate judgements, Ian McEwan argues, as he condemns the "utterly perverse and inhumane" decision of religious parents allowing their children to die.
McEwan, the author of Atonement, On Chesil Beach and Enduring Love, said he believed religion to be "distinctly unhelpful" in making rational choices.
His new book, The Children's Act, will explore the issues of the family courts, and the heartbreaking decisions faced when parents insist their child must die for religious reasons.
Speaking at the Oxford Literary Festival, McEwan used the examples of Jehovah's Witnesses who refused a blood transfusion for their son, and Catholic parents who insisted their conjoined twins must not be separated.
His plot, finished only last week, will follow the judge's decision on whether to save the stronger twin at the expense of his dying brother.
When asked whether he had learned anything from his time observing medical decisions in the family courts, he said: "I found generally that religion was distinctly unhelpful in making compassionate, reasonable judgments about people's lives.
"On the whole, the secular mind seems far superior in making reasonable judgments.
"Especially with the Jehovah's Witness, where parents allow their children to die because of some theological line invented in 1945 by a committee, seems to be utterly perverse and inhumane."
His new novella, finished only last week, was inspired in part by real life cases presided over by his own friends who worked as judges.
Speaking of his research in the family courts, McEwan told an audience he now believed the selfishness of some modern parents was ruining the interests of the children in their care.
Referring to divorce, custody, and abuse cases, he said: "There is human tragedy unfolding every day in a rather commonplace way.
"There's an almost consumerist notion that the pursuit of individual happiness cuts across the interests of children."
An extract of the novel will reference children as "counters in a game, bargaining chips for use by mothers, objects of financial or emotional neglect by fathers", as well as the victims of "real, fantasised or cynically-invented charges of abuse usually by mothers, sometimes by fathers".
The novel also describes "greedy husbands vs greedy wives, manoeuvring like nations at the end of a war, grabbing from the ruins what spoils they could before the final withdrawal".
The title of the novel refers to the real Children's Act, which McEwan called a "remarkable and civilised piece of legislation".
It rules: "When a court determines any questions with respect to the upbringing of a child, the child's welfare shall be the court's paramount consideration."
In 1999, McEwan was the subject of a scandalous custody battle with his first wife over his two children. He was eventually awarded sole custody, after his estranged wife ran away with one of his sons to France in defiance of a court ruling and was fined for defamation.
This is not the first time McEwan has shared his thoughts on the influence of religion on medical decisions.
In 2011, he called for an overhaul of suicide laws to allow terminally ill patients to receive medical help to die. At the time, he urged Prime Minister David Cameron to ignore the "supernatural beliefs" of Christians and introduce new laws to end "unnecessary suffering".
The novel, The Children's Act, is due out this autumn.