The new Prince of Cambridge has become the fifth in line to the throne, pushing Prince Andrew out of the top six for the first time.
The Duke and Duchess of Cambridge's third child - a younger sibling to Prince George and Princess Charlotte - is now the Queen and the Duke of Edinburgh's sixth great-grandchild.
The boy, weighing 8lbs 7oz, is also a great-great-great-great-great-grandchild of Queen Victoria.
Following the baby's arrival, Prince Harry shifts down the line of succession to sixth place.
The Duke of York, who was born second in line, has moved to seventh and Princesses Beatrice and Eugenie to eighth and ninth.
Andrew falling out of the top six means he will no longer have to ask the Queen for permission to marry - unlike Charles, William, George, Charlotte and HarrShare
The first six people in the line of succession must have the Queen's consent before they can marry on May 19, as set out by the Succession to the Crown Act 2013.
The act's introduction also saw sex removed as a determining factor in the line of succession.
It has also been claimed that the royal baby has been born a commoner.
Royal historian Marlene Koenig told Town and Country: 'It sounds complicated, but in the UK, the only people who are not commoners are the Sovereign and peers of the realm, (people with titles like) Duke, Marquess, Earl, Viscount, and Baron.'
It was always thought that William and Kate would go on to have three children.
Kate is one of three and had a happy home life with her sister, Pippa Matthews, and brother, James Middleton, and is close to both of her siblings.
But William may need to adjust to caring for a newborn once again.
Kate joked in the months leading up to her due date that her husband was 'in denial' about having a third.
By having more than two children, William and Kate are following in the footsteps of the Queen and the Duke of Edinburgh, who went on to have four children - although there was a gap of 10 years between their second child, Anne, and third, Andrew.
As a sibling to both future king George and 'spare to the heir' Charlotte, the new baby is unlikely ever to be crowned sovereign.
The Duke of York is the Queen and the Duke of Edinburgh's third child, but when Andrew was born in 1960 he leapfrogged his older sister, Princess Anne, in the line of succession.
If the Cambridges' third child is a boy, he will no longer be allowed to jump ahead of older sister Charlotte in the line of succession.
Previously, under the ancient rules of male primogeniture, royal sons took precedence over their female siblings, even leapfrogging first-born royal daughters.
But a radical shake-up of the royal succession rules removed discriminatory male bias and came into force in March 2015, affecting babies born after October 28 2011.
The Succession to the Crown Act 2013 has already affected the Duke of Gloucester's granddaughters, Senna Lewis and Lyla Gilman, whose younger brothers, born in 2012, now follow them in the line of succession.
The new baby will be a prince or princess thanks to the Queen, who stepped in ahead of Prince George's birth to ensure all William's children would become HRHs with fitting titles.
The Queen issued a Letters Patent under the Great Seal of the Realm in December 2012 when Kate was just a few months pregnant, declaring 'all the children of the eldest son of the Prince of Wales should have and enjoy the style, title and attribute of Royal Highness with the titular dignity of Prince or Princess prefixed to their Christian names or with such other titles of honour'.
A Letters Patent in 1917, issued by George V, limited titles within the royal family, meaning daughters born to William or Kate would not have been an HRH but Lady (forename) Mountbatten-Windsor instead and second or later-born sons would also have lacked the HRH title and become Lord (forename) Mountbatten-Windsors rather than princes.