The third in line to the throne will be baptised by the Archbishop of Canterbury on October 23, but it now seems that only a handful of the couple's closest family and friends will witness the occasion at the Chapel Royal in St James's Palace.
The Duke and Duchess have already shown a determination to set their own rules, rather than being bound by royal tradition, such as their choice of the Chapel Royal instead of Buckingham Palace, where the Duke and his father were christened.
But the mission of some of the most senior members of the Royal family from the guest list is likely to have come as a surprise to those who will not be there.
The Princess Royal will instead fly to Canada for a series of engagements in and around Toronto, while the Countess of Wessex will spend the day carrying out official duties in Somerset, Devon and Bristol.
It is unclear whether Prince William's uncles the Earl of Wessex and the Duke of York, or their children, who are William's cousins, will be at the christening, but royal sources suggested they may not, adding that "there have been other occasions in the past where a royal baby's great aunts and uncles have not attended the christening".
Asked why the Princess Royal and the Countess of Wessex had not been invited, a spokesman for the Duke and Duchess of Cambridge would only say: "We are not giving details of the guest list at this stage. All we are saying is that it will be a small intimate affair with close family and others, which include godparents."
Among those who will definitely attend the ceremony will be the Queen and the Duke of Edinburgh, the Prince of Wales and the Duchess of Cornwall, Prince Harry and the Duchess's parents, Carole and Michael Middleton, and her siblings James and Pippa. Prince Harry and James and Pippa Middleton may also be godparents.
When Prince William was christened in 1982 there were more than 60 guests, though the Duke of York, who was serving in the Falklands, and Princess Margaret, who was on holiday in Italy, did not attend.
The Chapel Royal has personal memories for the Duke and the Duchess, as the Duke's mother Diana, Princess of Wales, lay at rest there following her death in 1997, and the Duchess was confirmed into the Church of England there before her marriage in 2011.