In a statement shared by Buckingham Palace on Monday, the monarch, 95, is said to be "delighted with the news."
WATCH: Princess Beatrice and Edoardo Mapelli Mozzi welcome first child
It read in full: "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.
"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."
The Queen and Prince Philip pictured with their great-grandchildren in 2018
The 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. The baby girl 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.
After the death of the Duke of Edinburgh in April, Buckingham Palace released a never-seen-before photograph of Prince Philip and the Queen with seven of their great-grandchildren at Balmoral Castle in 2018.
Beatrice and Edoardo are yet to formally announce the name of their baby girl, and could leave royal fans waiting for over a week like Princess Eugenie.
The tot is 11th in line to the throne after her mother, but will not have a royal title or carry out duties in future.
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