Prince William and Kate Middleton congratulated Meghan Markle and Prince Harry on the birth of their daughter

• Kate and William congratulated Meghan and Harry on the birth of their daughter.
• The couple welcomed Lilibet Diana Mountbatten-Windsor on Friday.
• The Cambridges didn't use the Sussexes' official titles in their social media posts.

The Duke and Duchess of Cambridge publicly congratulated the Duke and Duchess of Sussex on the arrival of their daughter, Lili.

Meghan Markle and Prince Harry welcomed Lilibet Diana Mountbatten-Windsor to the family on Friday in Santa Barbara, California, and announced the news on Sunday.

"We are all delighted by the happy news of the arrival of baby Lili. Congratulations to Harry, Meghan, and Archie," Prince William and Kate Middleton wrote on Instagram, alongside a photo of the couple and Archie, taken during Markle's pregnancy.

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Prince Charles and the Duchess of Cornwall also extended their congratulations, writing on Instagram: "Congratulations to Harry, Meghan, and Archie on the arrival of baby Lilibet Diana. Wishing them all well at this time."

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Both the Cambridges and Charles and Camilla dropped Markle and Harry's duke and duchess titles from their posts, instead referring to the couple by their first names.

Meanwhile, The Royal Family account - the official account for the Queen - referred to the couple by their titles.

"Congratulations to The Duke and Duchess of Sussex on the birth of Lilibet Diana! The Queen, The Prince of Wales and The Duchess of Cornwall, and The Duke and Duchess of Cambridge are delighted with the news," a palace spokesperson wrote in the post. "Lilibet is Her Majesty's 11th great-grandchild."

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Lili is eighth in line to the British throne after Archie, who became seventh in line upon his birth on May 6, 2019.

She is the granddaughter to Prince Charles, a niece to William and Middleton, and a younger cousin to the Cambridge children, Prince George, Princess Charlotte, and Prince Louis.

"She is more than we could have ever imagined, and we remain grateful for the love and prayers we've felt from across the globe," Harry and Markle said in a statement after the birth. "Thank you for your continued kindness and support during this very special time for our family."

It is not known when Lili will be introduced to the royal family for the first time.

It's thought Harry is currently in the process of repairing his relationship with William after they were photographed in conversation with one another at Prince Philip's funeral in April. It was their first public reunion since Harry and Markle's Oprah interview on March 7.

"There's a general feeling in the media and amongst the public that we'd like to see them reconciled. So everybody, whether it's true or not, is grasping at any kind of straws of reconciliation and building a straw house of hope," Princess Diana's biographer Andrew Morton previously told Insider.

"I think there's a lot of water that's got to go under the bridge before those two and their father are reconciled. You don't say that your brother's trapped and your father's trapped in the institution, which you were born into, and expect everyone to be all sweet," he added.

Harry spoke to Oprah about Charles and William's positions within the royal family, saying: "My father and my brother, they are trapped. They don't get to leave. And I have huge compassion for that."