Surprise Moment Unfolds as Meghan and Prince Harry Join Kate and Prince William After Queen's Death

Alberto Pezzali/AP/Shutterstock

Kate Middleton and Prince William — the new Prince and Princess of Wales — stepped out on Saturday for their first outing together since receiving their new titles, accompanied by Prince Harry and Meghan Markle in a sweet surprise.

The foursome took an emotional tour of the tributes to the late Queen Elizabeth and greeted well-wishers at Windsor Castle for about 40 minutes. They all left in the same car, driven by William. According to Kensington Palace, Prince William invited his brother and sister-in-law to join him and Kate on the outing.

William thought the walkabout to greet the crowds "was an important show of unity at an incredibly difficult time for the family," a royal source says.

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Another source adds that security was not expecting both couples at the walkabout.

A palace insider tells PEOPLE: "It's such an extraordinary historical moment and also a deeply personal one for the family that you'd hope and think that all members of the family would unite and support [the King] especially. And perhaps some of those wounds can be healed in the process."

The couples delighted the crowds of people, who have flocked to the castle since news of the Queen's death was announced on Thursday.

Alberto Pezzali/AP/Shutterstock

RELATED: Queen Elizabeth, the Longest-Reigning British Monarch, Dies at 96

Kate and William, 40, recently moved to Windsor, making them a short distance from Harry and Meghan's U.K. home, Frogmore Cottage, which is on the Queen's Windsor Castle estate. Meghan, 41, and Harry, 37, who moved to California in 2020, are expected to remain in the U.K. until the Queen's funeral, which will take place on Sept. 19.

It was the second outing for William on Saturday. Earlier in the day, he witnessed the solemn historical moment that saw his father King Charles III officially proclaimed monarch at St. James's Palace in London.

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During the Accession Council of the Privy Council (comprising senior Cabinet ministers, judges and leaders of the Church of England) at St. James's Palace, it was proclaimed that "The crown of the United Kingdom of Great Britain and Northern Ireland is solely and rightfully come to the Prince Charles Philip Arthur George."

"We ... do now hereby with one voice and consent of tongue and heart publish and proclaim that the Prince Charles Philip Arthur George is now by the death of our late Sovereign of happy memory become our only lawful and rightful liege lord, Charles III."

James Whatling / MEGA Prince William and Prince Harry

William and Kate were named Prince and Princess of Wales on Friday by the King during his televised first address as monarch.

On Wednesday, the couple took their three children, Prince George, 9, Princess Charlotte, 7, and Prince Louis, 4, to their new school, Lambrook, after the family moved into Adelaide Cottage.

King Charles also acknowledged his younger son, who is now fifth in line to the throne, in the speech.

"I want also to express my love for Harry and Meghan as they continue to build their lives overseas," he said.

Alberto Pezzali/AP/Shutterstock Meghan Markle and Prince Harry

RELATED: Prince George, Princess Charlotte and Prince Louis Have New Names Under King Charles III

As news of concerns for the Queen's health broke on Thursday, William immediately made preparations to fly to Scotland alongside his uncles, Prince Andrew and Prince Edward, and Edward's wife Sophie, Countess of Wessex. Prince Harry also made his way to Scotland, having been in Europe for a scheduled appearance in London that day. But they weren't able to get to the Queen's bedside before she died.

Charles and Princess Anne, however, made it to Balmoral Castle in time to be by their mother's side before she died at age 96.

"The Queen died peacefully at Balmoral this afternoon," Buckingham Palace announced in an official statement on Thursday.

The statement continued, "The King and The Queen Consort will remain at Balmoral this evening and will return to London tomorrow."

James Whatling / MEGA

RELATED: King Charles Officially Names William and Kate Prince and Princess of Wales

In an official statement released Saturday, William said, "On Thursday, the world lost an extraordinary leader, whose commitment to the country, the Realms and the Commonwealth was absolute. So much will be said in the days ahead about the meaning of her historic reign."

"I, however, have lost a grandmother," he continued, reflecting on what the Queen meant to his family. "And while I will grieve her loss, I also feel incredibly grateful. I have had the benefit of The Queen's wisdom and reassurance into my fifth decade. My wife has had twenty years of her guidance and support. My three children have got to spend holidays with her and create memories that will last their whole lives."

"She was by my side at my happiest moments. And she was by my side during the saddest days of my life. I knew this day would come, but it will be some time before the reality of life without Grannie will truly feel real," said William.

PA From left: Prince George, Kate Middleton, Prince Louis, Prince William and Princess Charlotte

He went on to thank the late Queen "for the kindness she showed my family and me. And I thank her on behalf of my generation for providing an example of service and dignity in public life that was from a different age, but always relevant to us all."

William concluded, "My grandmother famously said that grief was the price we pay for love. All of the sadness we will feel in the coming weeks will be testament to the love we felt for our extraordinary Queen. I will honour her memory by supporting my father, The King, in every way I can."