The UK's Press Association said the decision is understood not to have been taken for health reasons and was mutually agreed by the Queen and Louis' parents, the Duke and Duchess of Cambridge, some time ago.
The Queen had a busy week last week in Sandringham, Norfolk, and she has another one ahead of her.
Will and Kate chose two hymns, two lessons and two anthems for the christening.
The event will be the first time that William, Kate and their three children have all been seen in public together.
Louis' other godparents are: Nicholas van Cutsem, whose family is close to the royals; William's school pal Harry Aubrey-Fletcher; Laura Meade, who is married to William's close friend James Meade; Kate's schoolfriend Hannah Carter; and the duchess's cousin Lucy Middleton.
She's a natural royal and seems to be a lot more confident than her shy older brother, Prince George.
Miss Middleton and Mr Pelly each read a lesson during the service, Kensington Palace said. She teamed it with a hairband-style hat by Jane Taylor, which swept her glossy chestnut hair back into her signature updo.
At the baptism of Prince George in 2013 at the same chapel, the Duchess of Cambridge wore a cream-coloured Alexander McQueen dress with a distinctive ruffle down the chest.
Princess Charlotte stole the show today as she saluted RAF pilots as they flew across the skies above Buckingham Palace.
The Royal Christening Robe was made in 1841 for the christening of Queen Victoria's eldest daughter, Victoria, Princess Royal. The original is now too delicate to be worn, so a copy has been made by the Queen's dresser Angela Kelly. The Lily Font and water from the River Jordan will be used during the baptism.
The Duke and Duchess of Cambridge with Prince Louis outside the Lindo Wing at St Mary's Hospital in Paddington, London.
Prince Louis was born on April 23, 2018, and on July 9, he was christened for the world to see.
Prince Louis held by his mother Kate Middleton at his royal christening.
Hours before the ceremony, Prince Louis' godparents were revealed in an official statement from the palace.