However, the decision for them not to attend Louis' baptism at St. James's Palace in London was understood to have been mutually agreed by the sovereign and the Duke and Duchess of Cambridge some time ago, and was not health-related.
The 11-week-old is set to get baptized at the Chapel Royal in St James's Palace and Kensington Palace have now confirmed that Nicholas van Cutsem, Guy Pelly, Harry Aubrey-Fletcher, Lady Laura Meade, Mrs. Robert Carter and Lucy Middleton will all be godparents.
Kate Middleton stunned, once again, at the christening of her third child, Louis, in London on July 9.
See how Prince Louis has grown!
Max Mumby/Indigo via Getty Images Prince Louis was born on April 23, 2018 in London, England.
From the duchess's side, Kate's parents Michael and Carole, brother James, pregnant sister Pippa and her husband James Matthews were there.
Monday's christening is a particularly special day as it was the first time the Cambridges have appeared together as a family of five.
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The tradition of this particular christening gown harkens back to Queen Victoria, who commissioned the garment for the royal baptism of her daughter Victoria in 1841; you can glimpse one of its earliest representations in a painting of the event from the era.
Charlotte, who's fourth in line to the throne, is already excelling at being an older sibling.
The ceremony will be in front of 40 close friends and family.
"Robinson Crusoe was William's favourite book, but Harry loved all things A.A. Milne", the source added.
His elder brother George was also christened at the Chapel Royal, but sister Charlotte's christening was at the Church of St Mary Magdalene in Sandringham.
The six godparents, who also attended with their spouses, are all friends or family of William and Kate.