Despite still being on maternity leave after giving birth to Prince Louis only 11 weeks ago, it is understood her mother Kate felt it was "important" to attend today's celebrations.
In addition to the cake, Prince Louis's christening gown was also recycled.
Will and Kate chose two hymns, two lessons and two anthems for the christening.
Other notable guests there to toast Prince Louis were Prince Charles and Camilla, Kate's parents Carole and Michael Middleton, her sister Pippa and her husband James Matthews, and hot bearded Middleton brother James.More news: Prince Harry and Meghan Markle attend Prince Louis' christening
Kate also remained loyal to her classic look, wearing an elegant McQueen dress once again for the occasion.
For the ceremony, Prince Louis will wear the handmade replica of the Royal Christening Robe, made by Angela Kelly, Dressmaker to The Queen.
Queen Elizabeth, 92, has also begun to scale back her duties because of her advancing age, handing over some of her responsibilities to her heir, Prince Charles.
Harry's hope is to establish a library for Louis, who was born on April 23, in tribute to the baby's late grandmother, Princess Diana, The Sun claims. Like his siblings Prince George and Princess Charlotte before him, Louis will be dressed up in an article of clothing that carries the weight of history.More news: YouTube spends $25 million to fight fake news
At her christening on July 5, 2015 at the Church of St. Mary Magdalene on the Sandringham Estate in Norfolk, Princess Charlotte wore the very same christening gown.
While they have not been named, William's private secretary Miguel Head - who leaves the post this month - is the bookies' favourite to be among the godparents.
Guy Pelly: The son of wealthy Kent landowners, Mr Pelly used to help run several London nightclubs. While Princes William and Harry looked smart in coordinating navy blue suits. He has been banned from driving three times, including twice for drink-driving and once for speeding. Van Cutsem is one of four brothers who have been close to William and Harry since they were little.
The Duke and Duchess have asked six close friends and family members to become Louis' godparents.More news: Fortnite Season 5 starting on July 12
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