The Prince of Wales invited the Duke and Duchess of Sussex to join him and the Princess of Wales on their walkabout outside Windsor Castle on Saturday.
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William and Harry were joined by their wives Kate and Meghan and there was a stunned silence out the castle grounds when the four left their vehicle together.
WATCH: Prince and Princess of Wales and the Duke and Duchess of Sussex unite at Windsor for emotional walkabout
In the wake of the Queen's death on Thursday, Prince William and Kate - who are now known as Prince and Princess of Wales - made an appearance in Winsdor and they greeted royal fans as they congregated to mourn the loss of the 96-year-old monarch who died "peacefully" in Balmoral.
Dressed in black, the royals looked sombre as they stopped to shake the hands of teary fans who were standing behind a barrier.
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They were surrounded by police protection as they made their way around the crowds.
They spent time together looking at the bouquets of flowers before they split off to talk to wellwishers. The group spent more than 40 minutes speaking to members of the public, where they received flowers, gifts and hugs.
Harry wrapped an arm around Meghan
Both of the royal ladies opted for smart black dresses, while Princes William and Harry donned black suits.
Crowds broke into applause seeing the royals couples together with the Princess of Wales approaching a crying child who stopped when she grew nearer.
The four appeared together
Buckingham Palace confirmed the sad news that the royal had passed away "peacefully at Balmoral" on Thursday 8 September after revealing her doctors were "concerned" for her health.
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Her Majesty's closest family were spotted rushing to her bedside, with Prince Harry cancelling an appearance at the WellChild awards to fly to Balmoral to join the likes of Prince William and now-King Charles III.
On Saturday, Buckingham Palace confirmed that Queen Elizabeth II's funeral will take place on 19 September at 11am with the service taking place at Westminster Abbey.