It's a girl.
The Duchess of Cambridge has given birth to a daughter, giving the Queen her fifth great-grandchild and installing a new princess fourth in line to the British throne.
Supporters outside St Mary's Hospital in London cheered when Kensington Palace announced that Kate "was safely delivered of a daughter at 8.34am" on Saturday morning (17.34 AEST).
The baby weighs 8lbs 3oz (3.7kg) and the Duchess's husband, Prince William, was present for the birth, which happened less than three hours after the Duchess was admitted to hospital.
"The Queen, the Duke of Edinburgh, the Prince of Wales, the Duchess of Cornwall, Prince Harry and members of both families have been informed and are delighted with the news," the palace said in a brief statement that was also released on Twitter.
"Her Royal Highness and her child are both doing well."
The name of the baby will be announced "in due course". Punters are favouring Alice, Charlotte or Elizabeth.
The new princess is fourth in line to the British throne behind Prince Charles, Prince William and her older brother, Prince George, who was also born at the private Lindo Wing in 2013.
Prince Harry, now in Australia on an army posting, has been bumped down to fifth.
The world is now waiting for the royal couple to emerge onto the hospital steps to show their new child.
It is expected William and Kate will pose for photographers with the princess just as they did 22 months ago with Prince George.
Sources say that could happen as soon as Saturday night (Sunday morning AEST).
Australian-born superfan Kathy Martin has been camped outside the Lindo Wing for almost a fortnight waiting for the royal baby to arrive.
She has been a royal supporter since seeing Princess Diana in Bendigo in 1983.
"I didn't camp out then because I didn't know about camping out at that time," Ms Martin told AAP while sitting next to her tent.
But after moving to Britain in 1996, she has camped out for Princess Diana's funeral, the Queen Mother's funeral, William and Kate's wedding, the Queen's diamond jubilee and Prince George's birth.
Ms Martin said on Saturday the arrival of a new princess was "awesome news".
Her voice hoarse from doing non-stop interviews, the 52-year-old said she had managed to preserve enough "so I can cheer them on".
Wrapped in an Australian flag, the royalist dismissed any suggestion Australia might one day become a republic, stating: "They won't get rid of the monarchy because there's strong royal supporters out there."
Ms Martin is tipping the princess will be named Elizabeth.
Fellow superfan Terry Hutt - who dresses in a loud Union Jack suit but is softly spoken - hopes the baby is called Amber. That's his granddaughter's name.
The 80-year-old is already looking forward to future births.
"It's wonderful because she (the new princess) is going to make more babies," the retired carpenter told AAP.
Australian Prime Minister Tony Abbott, who knighted the Queen's husband Prince Philip this year, said on Saturday "Australia shares this young family's joy".
"We wish the young princess a long and happy life," Mr Abbott said in a statement.
The Duchess of Cambridge was admitted to St Mary's Hospital at 6am local time. William and Kate travelled by car from Kensington Palace.
They will return to the palace for a few days after leaving the Lindo but then retreat to their Norfolk home Anmer Hall.
A paper proclamation announcing the new baby's birth has been placed on a golden easel outside Buckingham Palace and will remain there for 24 hours.
The build-up to this royal birth has not been as intense as two years ago mainly because the baby is a "spare" rather than a future heir.
Britain is also in the midst of a general election campaign.
