Inside the '11th hour' call that led to William, Harry, Kate and Meghan's surprise reunion

It was the royal reunion no one expected, as Prince William and wife Kate emerged from Windsor Castle alongside Prince Harry and wife Meghan.

And according to royal commentator Omid Scobie it almost didn't happen, as the call for the Duke and Duchess of Sussex to join the outing came at the last minute.

"The Waleses had always been scheduled to greet well-wishers at Windsor Castle, but royal sources say the decision to invite the Sussexes was made in the eleventh hour," the royal author wrote on Twitter.

READ MORE: William, Harry, Kate and Meghan reunite

"It is, without a doubt, a significant moment in the history of the relationship between the two brothers."

Only the newly appointed Prince and Princess of Wales were expected to meet the public from 4.30pm on Saturday, but the walkabout was delayed until 5.15pm in order to include Harry and Meghan.

Another source told The Sun it was William himself who extended the invitation, as the prince thought "it was an important show of unity for the Queen".

It was the first time Prince William and Kate, and Prince Harry and Meghan, have been together, in public, since the Commonwealth Day service in 2020.

That event marked Prince Harry and Meghan's final royal engagement before they began a new life as non-working royals in the United States.

READ MORE: Meghan hugs teenager on Windsor walkabout following Queen's death

Saturday's walkabout marked Kate's first public appearance as Princess of Wales since the title was bestowed upon the couple by King Charles III on Friday.

"Just as we saw after the death of Prince Philip and at his funeral, these are the moments when members of the royal family can put differences to one side to focus on the loss that has brought them together," Scobie added on Twitter.

"Today's public reunion was a show of the utmost respect to the Queen."

READ MORE: King Charles 'told Harry not to bring Meghan to Balmoral'

It comes as other senior royals emerged from Balmoral Castle for first time since the Queen's death to view flowers and tributes stacked at the castle gates.

The Queen's granddaughter Princess Eugenie broke down in tears, as she was comforted by her father Prince Andrew and sister Princess Beatrice.

Eugenie is yet to share any public tribute for Queen Elizabeth after she died on Thursday at the age of 96.

The royals left Balmoral to attend a prayer service at the nearby Crathie Kirk church.

Princess Anne, Prince Edward, Sophie, Countess of Wessex, as well as Zara Tindall and Lady Louise Windsor were also among the mourners.

Anne's husband, Vice Admiral Sir Tim Laurence, and Peter Phillips were also in attendance.