Speaking from the Nave, Diana, Princess of Wales' older sister will read from the Song of Solomon, which stresses the strength and power of love.
The short reading proclaims: "My beloved speaks and says to me: 'Arise, my love, my fair one, and come away; for now the winter is past, the rain is over and gone'."
The decision to give the important role of delivering the reading to the late princess's sibling shows the strong connection Harry retains with his mother's family.
Lady Jane, whose husband Baron Robert Fellowes is a former private secretary to the Queen, also gave a reading at Diana's funeral in 1997.
The passage that will be read on Saturday says: "Set me as a seal upon your heart, as a seal upon your arm; for love is as strong as death, passion fierce as the grave.
"Its flashes are flashes of fire, a raging flame. Many waters cannot quench love, neither can floods drown it."
It also says: "If one offered for love all the wealth of one's house, it would be utterly scorned."
The reading comes from the New Revised Standard Version of the Bible.
Harry was just 12 years old when his mother was killed in a Paris car crash in 1997.
The image of the young prince and brother William, then 15, walking behind Diana's coffin on the day of her funeral became etched in the public's memory.
All three of Diana's siblings - Earl Spencer, Lady Sarah McCorquodale and Lady Jane - will be at the royal wedding in Windsor.
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