In the letter, sent to a child fan in 1961, Lewis writes: “The whole Narnian story is about Christ.” It has been found by Walter Hooper, literary adviser to the Lewis estate.

It has emerged ahead of this week’s release of The Chronicles of Narnia: The Lion, The Witch and The Wardrobe. The film, starring Tilda Swinton and Jim Broadbent, cost £75m to make and has been at the centre of a tug of war between Christians and secularists.

Brian Sibley, author of Shadowlands, the book which describes Lewis’s marriage to Joy Gresham, said: “This is the most specific explanation of Narnia I have heard.”

The new film depicts one of the seven novels in Lewis’s series, which tell the story of four children journeying through a wardrobe into Narnia, a world of talking animals that is plunged into endless winter by a witch. The children and animals rally to Aslan, a noble lion.

On one side church groups, backed by the film’s producer Disney, are promoting the story’s message as Christian, with Jesus represented by Aslan saving a world fallen into sin.

Others say it is just an adventure story that draws on a variety of religious and folklore sources. Douglas Gresham, Lewis’s stepson, said recently: “Churches in Britain and America are promoting the film as a Christian film, but it’s not . . . and the Narnia books aren’t Christian novels.”

The letter, written from Magdalene College, Cambridge, where Lewis was a don, contradicts this. “Supposing there really was a world like Narnia . . . and supposing Christ wanted to go into that world and save it (as He did ours) what might have happened?” he wrote.

“The stories are my answer. Since Narnia is a world of talking beasts, I thought he would become a talking beast there as he became a man here. I pictured him becoming a lion there because a) the lion is supposed to be the king of beasts; b) Christ is called ‘the lion of Judah’ in the Bible.”

The text is contained in a volume of Lewis letters to be published next year.

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