As with her previous two children, Kate chose an all cream ensemble by Alexander McQueen, with a Jane Taylor fascinator, and beamed as she carried her youngest to the venue, while William held hands with George and Charlotte.
The Queen and Prince Phillip will not be attending the event, as Her Majesty is travelling back from Norfolk today.
There they would be joined by the Duke and Duchess of Cornwall and the Duke and Duchess of Sussex along with other guests.
The Royal Christening Robe was made in 1841 for the christening of Queen Victoria's eldest daughter, Victoria, Princess Royal.
England anthem ‘Three Lions’ got one million Spotify streams on Saturday alone
He said the England and Spurs defender the will treat the World Cup semi-final and any eventual final like a normal game. It's fantastic because I was one of those kids growing up who wanted to play for England . "No-one wants to go home".
A third snap captures a proud Princess Diana beaming down at her first son, Prince William, watched by the Queen and the Queen Mother on the now Duke of Cambridge's christening day in August 1982. 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.
Duke and Duchess of Cambridge with Prince George, Princess Charlotte and Prince Louis.
Miss Lucy Middleton is one of Kate, Pippa and James Middleton's cousins.
They include Guy Pelly, once dubbed the "court jester" to William and Harry for his wild ways that were often cited as a bad influence on the brothers in their youth, running their favourite drinking haunts. As the palace further explains, "the original gown was subsequently worn for all Royal christenings, including The Queen, her children and her grandchildren".
Why did William and Kate chose St James's Palace for Louis' christening?
Louis Arthur Charles weighed 8lb 7oz when he was born in the private Lindo Wing of St Mary's Hospital in Paddington, London, on 23 April.