Why Queen Elizabeth and Prince Philip weren't at Prince Louis' christening.

-With AAP

Duke and Duchess of Cambridge’s third child, Prince Louis, was christened in front of friends and family on Monday – but Queen Elizabeth and Prince Philip weren’t two of them.

The Queen and the Duke of Edinburgh had decided not to attend their great-grandson’s christening. A decision that Buckingham Palace says was made some time ago and agreed to by Prince William and Kate.

The 92-year-old monarch has a busy week ahead, marked by a visit from US President Donald Trump in Windsor on Friday.

The Duke of Edinburgh, 97, has retired from public duties and has been enjoying his free time away from London.

But Prince Louis was not without love and attention on his big day.

Prince George and Princess Charlotte were there to see their 11-week-old baby brother's christening at Chapel Royal, St James’s Palace, as were Prince Charles and his wife Camilla and Prince Harry and Meghan, Duchess of Sussex.

Not to mention Louis' six godparents.

Kensington Palace announced the tiny prince's godparents included Prince William’s longtime friend Guy Pelly. Pelly is a nightclub owner whose mother was close to Princess Diana, and he is known for having planned Prince William's Bachelor party.

Kate's older cousin, Lucy Middleton, is a godmother. She is a solicitor who also has a degree in French and Italian. She works as a senior lawyer at book publishing giant Penguin Random House. Kate visited Lucy in Florence during her gap year.

Also godparents are Kate's school friend Hannah Carter (née Gillingham), William's childhood friend Nicholas van Cutsem, his Eton College classmate Harry Aubrey-Fletcher, and close friend of the couple Lady Laura Meade.

Louis wore the handmade replica of the Royal Christening Robe, which in 2008 replaced the original which had been worn for all royal christenings since that of Queen Victoria's eldest daughter in 1841.

After the ceremony, guests were reportedly served (defrosted, we hope) slices from a tier of William and Kate's wedding cake during an afternoon tea.