Prince Louis, the third child of the Duke and Duchess of Cambridge, will be christened in London, but his great-grandparents won't be there.
Buckingham Palace say the Queen and Duke of Edinburgh will miss Monday's christening in a decision made some time ago and mutually agreed by the Queen and the Cambridges some time ago.
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 siblings Prince George and Princess Charlotte will be there to see 11-week-old Louis' big day, as will Prince Charles and his wife Camilla and Prince Harry and his wife Meghan.
Also attending are the Duchess of Cambridge's parents, her brother and sister and Louis's six godparents, who are close friends or relatives of William and Kate.
They include Kate's cousin Lucy Middleton and Guy Pelly, a childhood friend of William's.
"I'm delighted and privileged to be christening Prince Louis today - a precious child made in God's image, just as we all are," Archbishop of Canterbury Justin Welby said on Twitter. "Please join me in praying for him and his family on this special day."
Louis will wear 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.