In a statement, the palace said: "Her Royal Highness Princess Beatrice and Mr Edoardo Mapelli Mozzi are delighted to announce the safe arrival of their daughter on Saturday 18th September 2021, at 23.42, at the Chelsea and Westminster Hospital, London.
"The baby weighs 6 pounds and 2 ounces.
"The new baby's grandparents and great-grandparents have all been informed and are delighted with the news. The family would like to thank all the staff at the hospital for their wonderful care.
WATCH: Princess Beatrice and Edoardo Mapelli Mozzi welcome first child
"Her Royal Highness and her child are both doing well, and the couple are looking forward to introducing their daughter to her big brother Christopher Woolf."
Sarah is "over the moon" to welcome her new granddaughter, HELLO! can exclusively reveal. A friend of the family said the Duchess returned from Balmoral on Friday and met her new grandchild over the weekend. "She is absolutely over the moon," the friend added.
The couple at Wimbledon in July
The tot is 11th in line to the throne, meaning that Princess Eugenie will move down to 12th place. Eugenie and Jack Brooksbank's son, August, will also move further down in the line of succession to 13th.
While the baby girl is Beatrice's first child, property developer Edoardo already has a five-year-old son Christopher Woolf (affectionately known as Wolfie), from his previous relationship with American architect Dara Huang.
In a rare tweet from her personal account, Beatrice wrote: "So delighted to share the news of the safe arrival of our daughter on Saturday 18th September 2021, at 23.42, at the Chelsea and Westminster Hospital, London. Thank you to the Midwife team and everyone at the hospital for their wonderful care."
Beatrice's uncle, the Prince of Wales and his wife the Duchess of Cornwall, were also among the royals to react to the happy news, writing on Twitter: "Congratulations to Princess Beatrice and Edoardo Mapelli Mozzi on the happy news of the birth of their daughter."
Beatrice welcomed her first child at Chelsea and Westminster Hospital
Meanwhile, Prince William and Kate reshared the Queen's official message on their Twitter account. Proud father-of-two Edoardo also reposted Beatrice's message and the Queen's statement following the arrival.
Proud sister and new aunt Eugenie wrote in a touching Instagram post: "To my dearest Beabea and Edo, Congratulations on your new angel. I can't wait to meet her and I'm so proud of you. We're going to have so much fun watching our children grow up. Love Euge
"To my new niece, I love you already and think you're just awesome from the photos.. we're going to have so much fun together. Love your Auntie Euge."
What is the royal baby's name?
The couple are yet to announce her name, and they could decide to wait over a week to confirm the moniker, like Princess Eugenie.
Sarah is the early favourite for the baby's name at 8/1 with Betfair.
Betfair spokesperson Sam Rosbottom said: "Royal family fans will be delighted to hear the news that not long after sister Eugenie gave birth, Beatrice has welcomed her first child to the world, a bouncing baby girl - and Sarah is the early 8/1 favourite, a nod to her mother Sarah the Duchess of York.
"We are tipping the happy couple to have an Italian flavour with their choice of name, with Edoardo's father hailing from Bergamo. The likes of Anna at 10/1 second favourite, Maria at 12/1 and Rosa at 14/1 suggest they could break from traditional British names usually favoured by the Royal Family."
Ladbrokes, meanwhile, are also seeing punters betting on an Italian name, with Matilda leading the field at 4/1, closely followed by Florence at 9/2. Arabella has odds of 5/1 as does Cecelia, while Giovanna is 12/1. Elizabeth after the Queen is 7/1.
Similarly, William Hill has Florence at 9/2 and Francesca at 5/1. Elizabeth has halved in price from 20/1 to 10/1.
Will the royal baby have a title?
As things currently stand, the baby will not have a royal title because these are passed down the male line.
However, the Queen could decide to intervene and give Beatrice's husband an earldom or issue a Letters Patent amending the rules.
Beatrice and Edoardo have welcomed a baby girl
But it's unlikely that Beatrice and Edoardo's baby girl will have a royal title, as their children will take their rank from their father.
Edoardo's official title is Count, as his family was part of the Italian nobility before the country became a republic back in 1946.
Although they do not use their titles in everyday life, his family retains them.
That means that the couple's baby will be passed on the title of Nobile Donna (Noble Woman).
Where will the royal baby live?
Beatrice has an apartment at St James's Palace in London, but the couple have also reportedly purchased a £3million family home in the Cotswolds.
When did Princess Beatrice marry Edoardo Mapelli Mozzi?
The couple tied the knot in a private ceremony at the Royal Chapel of All Saints at Royal Lodge in Windsor on 17 July 2020. Originally they planned to wed at the Chapel Royal at St James's Palace in May 2020 but they were forced to postpone their nuptials due to the pandemic.
Beatrice and Edoardo on their wedding day
Princess Beatrice looked stunning in one of the Queen's vintage Norman Hartnell gowns for her big day, and was loaned Queen Mary's diamond fringe tiara.
Who are the Queen's great-grandchildren?
Beatrice and Edoardo's baby girl is the fourth great-grandchild the Queen has welcomed in 2021. Princess Eugenie and Jack Brooksbank had their son, August, in February, while Zara and Mike Tindall's third child, Lucas, arrived in March.
The Duchess of Sussex also gave birth to her and Prince Harry's second child, Lilibet Diana Mountbatten-Windsor, at Santa Barbara Cottage Hospital in June. Their daughter was named in honour of the Queen's family nickname.
Her Majesty is also a great-grandmother to Peter and Autumn Phillips' daughters, Savannah, ten, and Isla, nine, as well as Zara and Mike's daughters, Mia, seven, and Lena, three, Meghan and Harry's son, Archie, two, and the Duke and Duchess of Cambridge's children, Prince George, eight, Princess Charlotte, six, and three-year-old Prince Louis.
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